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<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix Bottleneck Analysis</h1>


<h2>Purpose of this document </h2>

<p> This document describes the qshape(1) program which helps the
administrator understand the Postfix queue message distribution
sorted by time and by sender or recipient domain. qshape(1) is
bundled with the Postfix 2.1 source under the "auxiliary" directory.

<p> In order to understand the output of qshape(1), it useful to
understand the various Postfix queues. To this end the role of each
Postfix queue directory is described briefly in the "Background
info:  Postfix queue directories" section near the end of this
document.  </p>

<p> This document covers the following topics: </p>


<li><a href="#qshape">Introducing the qshape tool</a>

<li><a href="#trouble_shooting">Trouble shooting with qshape</a> 

<li><a href="#healthy">Example 1: Healthy queue</a>

<li><a href="#dictionary_bounce">Example 2: Deferred queue full of
dictionary attack bounces</a></li>

<li><a href="#active_congestion">Example 3: Congestion in the active

<li><a href="#backlog">Example 4: High volume destination backlog</a>

<li><a href="#queues">Background info: Postfix queue directories</a>


<li> <a href="#maildrop_queue"> The "maildrop" queue </a>

<li> <a href="#hold_queue"> The "hold" queue </a>

<li> <a href="#incoming_queue"> The "incoming" queue </a>

<li> <a href="#active_queue"> The "active" queue </a>

<li> <a href="#deferred_queue"> The "deferred" queue </a>


<li><a href="#credits">Credits</a>


<h2><a name="qshape">Introducing the qshape tool</a></h2>

<p> When mail is draining slowly or the queue is unexpectedly large,
run qshape(1) as the super-user (root) to help zero in on the problem.
The qshape(1) program displays a tabular view of the Postfix queue
contents.  </p>


<li> <p> On the horizontal axis, it displays the queue age with
fine granularity for recent messages and (geometrically) less fine
granularity for older messages.  </p>

<li> <p> The vertical axis displays the destination (or with the
"-s" switch the sender) domain. Domains with the most messages are
listed first. </p>


<p> For example, in the output below we see the top 10 lines of
the (mostly forged) sender domain distribution for captured spam
in the "hold" queue: </p>

$ qshape -s hold | head
                         T  5 10 20 40 80 160 320 640 1280 1280+
                 TOTAL 486  0  0  1  0  0   2   4  20   40   419
             yahoo.com  14  0  0  1  0  0   0   0   1    0    12
  extremepricecuts.net  13  0  0  0  0  0   0   0   2    0    11
        ms35.hinet.net  12  0  0  0  0  0   0   0   0    1    11
      winnersdaily.net  12  0  0  0  0  0   0   0   2    0    10
           hotmail.com  11  0  0  0  0  0   0   0   0    1    10
           worldnet.fr   6  0  0  0  0  0   0   0   0    0     6
        ms41.hinet.net   6  0  0  0  0  0   0   0   0    0     6
                osn.de   5  0  0  0  0  0   1   0   0    0     4


<li> <p> The "T" column shows the total (in this case sender) count
for each domain.  The columns with numbers above them, show counts
for messages aged fewer than that many minutes, but not younger
than the age limit for the previous column.  The row labeled "TOTAL"
shows the total count for all domains. </p>

<li> <p> In this example, there are 14 messages allegedly from
yahoo.com, 1 between 10 and 20 minutes old, 1 between 320 and 640
minutes old and 12 older than 1280 minutes (1440 minutes in a day).


<p> By default, qshape shows statistics for the union of both the
incoming and active queues which are the most relevant queues to
look at when analyzing performance. </p>

<p> One can request an alternate list of queues: </p>

$ qshape deferred | less
$ qshape incoming active deferred | less

<p> this will show the age distribution of the deferred queue or
the union of the incoming active and deferred queues. </p>

<p> Command line options control the number of display "buckets",
the age limit for the smallest bucket, display of parent domain
counts and so on. The "-h" option outputs a summary of the available
switches. </p>

<h2><a name="trouble_shooting">Trouble shooting with qshape</a>

<p> Large numbers in the qshape output represent a large number of
messages that are destined to (or alleged to come from) a particular
domain.  It should be possible to tell at a glance which domains
dominate the queue sender or recipient counts, approximately when
a burst of mail started, and when it stopped. </p>

<p> The problem destinations or sender domains appear near the top
left corner of the output table. Remember that the active queue
can accommodate up to 20000 ($qmgr_message_active_limit) messages.
To check wether this limit has been reached, use: </p>

$ qshape -s active | head       <i>(show sender statistics)</i>

<p> If the total sender count is below 20000 the active queue is
not yet saturated, any high volume sender domains show near the
top of the output.

<p> The active queue is also limited to at most 20000 recipient
addresses ($qmgr_message_recipient_limit). To check for exhaustion
of this limit use: </p>

$ qshape active | head          <i>(show recipient statistics)</i>

<p> Having found the high volume domains, it is often useful to
search the logs for recent messages pertaining to the domains in
question. </p>

# Find deliveries to example.com
$ tail -10000 /var/log/maillog |
        egrep -i ': to=&lt;.*@example\.com&gt;,' |

# Find messages from example.com
$ tail -10000 /var/log/maillog |
        egrep -i ': from=&lt;.*@example\.com&gt;,' |

<p> You may want to drill in on some specific queue ids: </p>

# Find all messages for a specific queue id.
$ tail -10000 /var/log/maillog | egrep ': 2B2173FF68: '

<p> Also look for queue manager warning messages in the log. These
warnings can suggest strategies to reduce congestion. </p>

$ egrep 'qmgr.*(panic|fatal|error|warning):' /var/log/maillog

<p> When all else fails try the Postfix mailing list for help, but
please don't forget to include the top 10 or 20 lines of qshape(1)
output.  </p>

<h2><a name="healthy">Example 1: Healthy queue</a></h2>

<p> When looking at just the incoming and active queues, under
normal conditions (no congestion) the incoming and active queues
are nearly empty. Mail leaves the system almost as quickly as it
comes in or is deferred without congestion in the active queue.

$ qshape        <i>(show incoming and active queue status)</i>

                 T  5 10 20 40 80 160 320 640 1280 1280+
          TOTAL  5  0  0  0  1  0   0   0   1    1     2
  meri.uwasa.fi  5  0  0  0  1  0   0   0   1    1     2

<p> If one looks at the two queues separately, the incoming queue
is empty or perhaps briefly has one or two messages, while the
active queue holds more messages and for a somewhat longer time:

$ qshape incoming

                 T  5 10 20 40 80 160 320 640 1280 1280+
          TOTAL  0  0  0  0  0  0   0   0   0    0     0

$ qshape active

                 T  5 10 20 40 80 160 320 640 1280 1280+
          TOTAL  5  0  0  0  1  0   0   0   1    1     2
  meri.uwasa.fi  5  0  0  0  1  0   0   0   1    1     2

<h2><a name="dictionary_bounce">Example 2: Deferred queue full of
dictionary attack bounces</a></h2>

<p> This is from a server where recipient validation is not yet
available for some of the hosted domains. Dictionary attacks on
the unvalidated domains result in bounce backscatter. The bounces
dominate the queue, but with proper tuning they do not saturate the
incoming or active queues. The high volume of deferred mail is not
a direct cause for alarm. </p>

$ qshape deferred | head

                         T  5 10 20 40 80 160 320 640 1280 1280+
                TOTAL 2234  4  2  5  9 31  57 108 201  464  1353
  heyhihellothere.com  207  0  0  1  1  6   6   8  25   68    92
  pleazerzoneprod.com  105  0  0  0  0  0   0   0   5   44    56
       groups.msn.com   63  2  1  2  4  4  14  14  14    8     0
    orion.toppoint.de   49  0  0  0  1  0   2   4   3   16    23
          kali.com.cn   46  0  0  0  0  1   0   2   6   12    25
        meri.uwasa.fi   44  0  0  0  0  1   0   2   8   11    22
    gjr.paknet.com.pk   43  1  0  0  1  1   3   3   6   12    16
 aristotle.algonet.se   41  0  0  0  0  0   1   2  11   12    15

<p> The domains shown are mostly bulk-mailers and all the volume
is the tail end of the time distribution, showing that short term
arrival rates are moderate. Larger numbers and lower message ages
are more indicative of current trouble. Old mail still going nowhere
is largely harmless so long as the active and incoming queues are
short. We can also see that the groups.msn.com undeliverables are
low rate steady stream rather than a concentrated dictionary attack
that is now over. </p>

$ qshape -s deferred | head

                     T  5 10 20 40 80 160 320 640 1280 1280+
            TOTAL 2193  4  4  5  8 33  56 104 205  465  1309
    MAILER-DAEMON 1709  4  4  5  8 33  55 101 198  452   849
      example.com  263  0  0  0  0  0   0   0   0    2   261
      example.org  209  0  0  0  0  0   1   3   6   11   188
      example.net    6  0  0  0  0  0   0   0   0    0     6
      example.edu    3  0  0  0  0  0   0   0   0    0     3
      example.gov    2  0  0  0  0  0   0   0   1    0     1
      example.mil    1  0  0  0  0  0   0   0   0    0     1

<p> Looking at the sender distribution, we see that as expected
most of the messages are bounces. </p>

<h2><a name="active_congestion">Example 3: Congestion in the active

<p> This example is taken from a Feb 2004 discussion on the Postfix
Users list.  Congestion was reported with the active and incoming
queues large and not shrinking despite very large delivery agent
process limits.  The thread is archived at:
http://groups.google.com/groups?th=636626c645f5bbde </p>

<p> Using an older version of qshape(1) it was quickly determined
that all the messages were for just a few destinations: </p>

$ qshape        <i>(show incoming and active queue status)</i>

                           T   A   5  10  20  40  80 160 320 320+
                 TOTAL 11775 9996  0   0   1   1  42  94 221 1420
  user.sourceforge.net  7678 7678  0   0   0   0   0   0   0    0
 lists.sourceforge.net  2313 2313  0   0   0   0   0   0   0    0
        gzd.gotdns.com   102    0  0   0   0   0   0   0   2  100

<p> The "A" column showed the count of messages in the active queue,
and the numbered columns showed totals for the deferred queue. At
10000 messages (Postfix 1.x active queue size limit) the active
queue is full. The incoming was growing rapidly. </p>

<p> With the trouble destinations clearly identified, the administrator
quickly found and fixed the problem. It is substantially harder to
glean the same information from the logs. While a careful reading
of mailq(1) output should yield similar results, it is much harder
to gauge the magnitude of the problem by looking at the queue
one message at a time. </p>

<h2><a name="backlog">Example 4: High volume destination backlog</a></h2>

<p> When a site you send a lot of email to is down or slow, mail
messages will rapidly build up in the deferred queue, or worse, in
the active queue. The qshape output will show large numbers for
the destination domain in all age buckets that overlap the starting
time of the problem: </p>

$ qshape deferred | head

                    T   5  10  20  40   80  160 320 640 1280 1280+
           TOTAL 5000 200 200 400 800 1600 1000 200 200  200   200
  highvolume.com 4000 160 160 320 640 1280 1440   0   0    0     0

<p> Here the "highvolume.com" destination is continuing to accumulate
deferred mail. The incoming and active queues are fine, but the
deferred queue started growing some time between 1 and 2 hours ago
and continues to grow. </p>

<p> If the high volume destination is not down, but is instead
slow, one might see similar congestion in the active queue. Active
queue congestion is a greater cause for alarm; one might need to
take measures to ensure that the mail is deferred instead or even
add an access(5) rule asking the sender to try again later. </p>

<p> If a high volume destination exhibits frequent bursts of
consecutive connections refused by all MX hosts or "421 Server busy
errors", it is possible for the queue manager to mark the destination
as "dead" despite the transient nature of the errors. The destination
will be retried again after the expiration of a $minimal_backoff_time
timer.  If the error bursts are frequent enough it may be that only
a small quantity of email is delivered before the destination is
again marked "dead". </p>

<p> The MTA that has been observed most frequently to exhibit such
bursts of errors is Microsoft Exchange, which refuses connections
under load. Some proxy virus scanners in front of the Exchange
server propagate the refused connection to the client as a "421"
error. </p>

<p> Note that it is now possible to configure Postfix to exhibit
similarly erratic behavior by misconfiguring the anvil(8) server
(not included in Postfix 2.1.). Do not use anvil(8) for steady-state
rate limiting, its purpose is DoS prevention and the rate limits
set should be very generous! </p>

<p> In the long run it is hoped that the Postfix dead host detection
and concurrency control mechanism will be tuned to be more "noise"
tolerant.  If one finds oneself needing to deliver a high volume
of mail to a destination that exhibits frequent brief bursts of
errors, there is a subtle workaround. </p>


<li> <p> In master.cf set up a dedicated clone of the "smtp"
transport for the destination in question. </p>

<li> <p> In master.cf configure a reasonable process limit for the
transport (a number in the 10-20 range is typical). </p>

<li> <p> IMPORTANT!!! In main.cf configure a very large initial
and destination concurrency limit for this transport (say 200). </p>

    initial_destination_concurrency = 200
    <i>transportname</i>_destination_concurrency_limit = 200

<p> Where <i>transportname</i> is the name of the master.cf entry
in question. </p>


<p> The effect of this surprising configuration is that up to 200
consecutive errors are tolerated without marking the destination
dead, while the total concurrency remains reasonable (10-20
processes). This trick is only for a very specialized situation:
high volume delivery into a channel with multi-error bursts
that is capable of high throughput, but is repeatedly throttled by
the bursts of errors.

<p> When a destination is unable to handle the load even after the
Postfix process limit is reduced to 1, a desperate measure is to
insert brief delays between delivery attempts. </p>


<li> <p>  In the transport map entry for the problem destination,
specify a dead host as the primary nexthop. </p>

<li> <p> In the master.cf entry for the transport specify the
problem destination as the fallback_relay and specify a small
smtp_connect_timeout value. </p>

    problem.example.com  slow:[dead.host]

    # service type  private unpriv  chroot  wakeup  maxproc command
    slow      unix     -       -       n       -       1    smtp
        -o fallback_relay=problem.example.com
        -o smtp_connect_timeout=1


<p> This solution forces the Postfix smtp(8) client to wait for
$smtp_connect_timeout seconds between deliveries. The solution
depends on Postfix connection management details, and needs to be
updated when SMTP connection caching is introduced.  </p>

<p> Hopefully a more elegant solution to these problems will be
found in the future. </p>

<h2><a name="queues">Background info: Postfix queue directories</a></h2>

<p> The following sections describe Postfix queues: their purpose,
what normal behavior looks like, and how to diagnose abnormal
behavior. </p>

<h3> <a name="maildrop_queue"> The "maildrop" queue </a> </h3>

<p> Messages that have been submitted via the Postfix sendmail(1)
command, but not yet brought into the main Postfix queue by the
pickup(8) service, await processing in the "maildrop" queue. Messages
can be added to the "maildrop" queue even when the Postfix system
is not running. They will begin to be processed once Postfix is
started.  </p>

<p> The "maildrop" queue is drained by the single threaded pickup(8)
service scanning the queue directory periodically or when notified
of new message arrival by the postdrop(1) program. The postdrop(1)
program is a setgid helper that allows the unprivileged Postfix
sendmail(1) program to inject mail into the "maildrop" queue and
to notify the pickup(8) service of its arrival. </p>

<p> All mail that enters the main Postfix queue does so via the
cleanup(8) service. The cleanup service is responsible for envelope
and header rewriting, header and body regular expression checks,
automatic bcc recipient processing and guaranteed insertion of the
message into the Postfix "incoming" queue. </p>

<p> In the absence of excessive CPU consumption in cleanup(8) header
or body regular expression checks or other software consuming all
available CPU resources, Postfix performance is disk I/O bound.
The rate at which the pickup(8) service can inject messages into
the queue is largely determined by disk access times, since the
cleanup(8) service must commit the message to stable storage before
returning success. The same is true of the postdrop(1) program
writing the message to the "maildrop" directory. </p>

<p> As the pickup service is single threaded, it can only deliver
one message at a time at a rate that does not exceed the reciprocal
disk I/O latency (+ CPU if not negligible) of the cleanup service.

<p> Congestion in this queue is indicative of an excessive local
message submission rate or perhaps excessive CPU consumption in
the cleanup(8) service due to excessive body_checks. </p>

<p> Note, that once the active queue is full, the cleanup service
will attempt to slow down message injection by pausing $in_flow_delay
for each message. In this case "maildrop" queue congestion may be
a consequence of congestion downstream, rather than a problem in
its own right. </p>

<p> Note also, that one should not attempt to deliver large volumes
of mail via the pickup(8) service. High volume sites must avoid
using content filters that reinject scanned mail via Postfix
sendmail(1) and postdrop(1). </p>

<p> A high arrival rate of locally submitted mail may be an indication
of an uncaught forwarding loop, or a run-away notification program.
Try to keep the volume of local mail injection to a moderate level.

<p> The "postsuper -r" command can place selected messages into
the "maildrop" queue for reprocessing. This is most useful for
resetting any stale content_filter settings. Requeuing a large number
of messages using "postsuper -r" can clearly cause a spike in the
size of the "maildrop" queue. </p>

<h3> <a name="hold_queue"> The "hold" queue </a> </h3>

<p> The administrator can define "smtpd" access(5) policies, or
cleanup(8) header/body checks that cause messages to be automatically
diverted from normal processing and placed indefinitely in the
"hold" queue.  Messages placed in the "hold" queue stay there until
the administrator intervenes. No periodic delivery attempts are
made for messages in the "hold" queue. The postsuper(1) command
can be used to manually release messages into the "deferred" queue.

<p> Messages can potentially stay in the "hold" queue for a time
exceeding the normal maximal queue lifetime (after which undelivered
messages are bounced back to the sender). If such "old" messages
need to be released from the "hold" queue, they should typically
be moved into the "maildrop" queue, so that the message gets a new
timestamp and is given more than one opportunity to be delivered.
Messages that are "young" can be moved directly into the "deferred"
queue. </p>

<p> The "hold" queue plays little role in Postfix performance, and
monitoring of the "hold" queue is typically more closely motivated
by tracking spam and malware, than by performance issues. </p>

<h3> <a name="incoming_queue"> The "incoming" queue </a> </h3>

<p> All new mail entering the Postfix queue is written by the
cleanup(8) service into the "incoming" queue. New queue files are
created owned by the "postfix" user with an access bitmask (or
mode) of 0600. Once a queue file is ready for further processing
the cleanup(8) service changes the queue file mode to 0700 and
notifies the queue manager of new mail arrival. The queue manager
ignores incomplete queue files whose mode is 0600, as these are
still being written by cleanup.  </p>

<p> The queue manager scans the incoming queue bringing any new
mail into the "active" queue if the active queue resource limits
have not been exceeded. By default, the active queue accommodates
at most 20000 messages. Once the active queue message limit is
reached, the queue manager stops scanning the incoming (and deferred,
see below) queue.  </p>

<p> Under normal conditions the incoming queue is nearly empty (has
only mode 0600 files), with the queue manager able to import new
messages into the active queue as soon as they become available.

<p> The incoming queue grows when the message input rate spikes
above the rate at which the queue manager can import messages into
the active queue. The main factor slowing down the queue manager
is transport queries to the trivial-rewrite service. If the queue
manager is routinely not keeping up, consider not using "slow"
lookup services (MySQL, LDAP, ...) for transport lookups or speeding
up the hosts that provide the lookup service.  </p>

<p> The in_flow_delay parameter is used to clamp the input rate
when the queue manager starts to fall behind. The cleanup(8) service
will pause for $in_flow_delay seconds before creating a new queue
file if it cannot obtain a "token" from the queue manager.  </p>

<p> Since the number of cleanup(8) processes is limited in most
cases by the SMTP server concurrency, the input rate can exceed
the output rate by at most "SMTP connection count" / $in_flow_delay
messages per second.  </p>

<p> With a default process limit of 100, and an in_flow_delay of
1s, the coupling is strong enough to limit a single run-away injector
to 1 message per second, but is not strong enough to deflect an
excessive input rate from many sources at the same time.  </p>

<p> If a server is being hammered from multiple directions, consider
raising the in_flow_delay to 10 seconds, but only if the incoming
queue is growing even while the active queue is not full and the
trivial-rewrite service is using a fast transport lookup mechanism.

<h3> <a name="active_queue"> The "active" queue </a> </h3>

<p> The queue manager is a delivery agent scheduler; it works to
ensure fast and fair delivery of mail to all destinations within
designated resource limits.  </p>

<p> The active queue is somewhat analogous to an operating system's
process run queue. Messages in the active queue are ready to be
sent (runnable), but are not necessarily in the process of being
sent (running).  </p>

<p> While most Postfix administrators think of the "active" queue
as a directory on disk, the real "active" queue is a set of data
structures in the memory of the queue manager process.  </p>

<p> Messages in the "maildrop", "hold", "incoming" and "deferred"
queues (see below) do not occupy memory; they are safely stored on
disk waiting for their turn to be processed. The envelope information
for messages in the "active" queue is managed in memory, allowing
the queue manager to do global scheduling, allocating available
delivery agent processes to an appropriate message in the active
queue.  </p>

<p> Within the active queue, (multi-recipient) messages are broken
up into groups of recipients that share the same transport/nexthop
combination; the group size is capped by the transport's recipient
concurrency limit.  </p>

<p> Multiple recipient groups (from one or more messages) are queued
for delivery via the common transport/nexthop combination. The
destination concurrency limit for the transports caps the number
of simultaneous delivery attempts for each nexthop. Transports with
a recipient concurrency limit of 1 are special: these are grouped
by the actual recipient address rather than the nexthop, thereby
enabling per-recipient concurrency limits rather than per-domain
concurrency limits. Per-recipient limits are appropriate when
performing final delivery to mailboxes rather than when relaying
to a remote server.  </p>

<p> Congestion occurs in the active queue when one or more destinations
drain slower than the corresponding message input rate. If a
destination is down for some time, the queue manager will mark it
dead, and immediately defer all mail for the destination without
trying to assign it to a delivery agent. In this case the messages
will quickly leave the active queue and end up in the deferred
queue. If the destination is instead simply slow, or there is a
problem causing an excessive arrival rate the active queue will
grow and will become dominated by mail to the congested destination.

<p> The only way to reduce congestion is to either reduce the input
rate or increase the throughput. Increasing the throughput requires
either increasing the concurrency or reducing the latency of
deliveries.  </p>

<p> For high volume sites a key tuning parameter is the number of
"smtp" delivery agents allocated to the "smtp" and "relay" transports.
High volume sites tend to send to many different destinations, many
of which may be down or slow, so a good fraction of the available
delivery agents will be blocked waiting for slow sites. Also mail
destined across the globe will incur large SMTP command-response
latencies, so high message throughput can only be achieved with
more concurrent delivery agents.  </p>

<p> The default "smtp" process limit of 100 is good enough for most
sites, and may even need to be lowered for sites with low bandwidth
connections (no use increasing concurrency once the network pipe
is full). When one finds that the queue is growing on an "idle"
system (CPU, disk I/O and network not exhausted) the remaining
reason for congestion is insufficient concurrency in the face of
a high average latency. If the number of outbound SMTP connections
(either ESTABLISHED or SYN_SENT) reaches the process limit, mail
is draining slowly and the system and network are not loaded, raise
the "smtp" and/or "relay" process limits!  </p>

<p> Especially for the "relay" transport, consider lower SMTP
connection timeouts (1-5 seconds) and higher than default destination
concurrency limits. Compute the expected latency when 1 out of N
of the MX hosts for a high volume site is down and not responding,
and make sure that the configured concurrency divided by this
latency exceeds the required steady-state message rate. If the
destination is managed by you, consider load balancers in front of
groups of MX hosts. Load balancers have higher uptime and will be
able to hide individual MX host failures.  </p>

<p> If necessary, dedicate and tune custom transports for high
volume destinations.  </p>

<p> Another common cause of congestion is unwarranted flushing of
the entire deferred queue. The deferred queue holds messages that
are likely to fail to be delivered and are also likely to be slow
to fail delivery (timeouts). This means that the most common reaction
to a large deferred queue (flush it!) is more than likely counter-
productive, and is likely to make the problem worse. Do not flush
the deferred queue unless you expect that most of its content has
recently become deliverable (e.g. relayhost back up after an outage)!

<p> Note that whenever the queue manager is restarted, there may
already be messages in the active queue directory, but the "real"
active queue in memory is empty. In order to recover the in-memory
state, the queue manager moves all the active queue messages
back into the incoming queue, and then uses its normal incoming
queue scan to refill the active queue. The process of moving all
the messages back and forth, redoing transport table (trivial-rewrite(8)
resolve service) lookups, and re-importing the messages back into
memory is expensive. At all costs, avoid frequent restarts of the
queue manager.  </p>

<h3> <a name="deferred_queue"> The "deferred" queue </a> </h3>

<p> When all the deliverable recipients for a message are delivered,
and for some recipients delivery failed for a transient reason (it
might succeed later), the message is placed in the deferred queue.

<p> The queue manager scans the deferred queue periodically. The
scan interval is controlled by the queue_run_delay parameter.
While a deferred queue scan is in progress, if an incoming queue
scan is also in progress (ideally these are brief since the incoming
queue should be short), the queue manager alternates between bringing
a new "incoming" message and a new "deferred" message into the
queue.  This "round-robin" strategy prevents starvation of either
the incoming or the deferred queues.  </p>

<p> Each deferred queue scan only brings a fraction of the deferred
queue back into the active queue for a retry. This is because each
message in the deferred queue is assigned a "cool-off" time when
it is deferred.  This is done by time-warping the modification
times of the queue file into the future. The queue file is not
eligible for a retry if its modification time is not yet reached.

<p> The "cool-off" time is at least $minimal_backoff_time and at
most $maximal_backoff_time. The next retry time is set by doubling
the message's age in the queue, and adjusting up or down to lie
within the limits. This means that young messages are initially
retried more often than old messages.  </p>

<p> If a high volume site routinely has large deferred queues, it
may be useful to adjust the queue_run_delay, minimal_backoff_time
and maximal_backoff_time to provide short enough delays on first
failure, with perhaps longer delays after multiple failures, to
reduce the retransmission rate of old messages and thereby reduce
the quantity of previously deferred mail in the active queue.  </p>

<p> One common cause of large deferred queues is failure to validate
recipients at the SMTP input stage. Since spammers routinely launch
dictionary attacks from unrepliable sender addresses, the bounces
for invalid recipient addresses clog the deferred queue (and at
high volumes proportionally clog the active queue). Recipient
validation is strongly recommended through use of the local_recipient_maps
and relay_recipient_maps parameters.  </p>

<p> When a host with lots of deferred mail is down for some time,
it is possible for the entire deferred queue to reach its retry
time simultaneously. This can lead to a very full active queue once
the host comes back up. The phenomenon can repeat approximately
every maximal_backoff_time seconds if the messages are again deferred
after a brief burst of congestion. Ideally, in the future Postfix
will add a random offset to the retry time (or use a combination
of strategies) to reduce the chances of repeated complete deferred
queue flushes.  </p>

<h2><a name="credits">Credits</a></h2>

<p> The qshape(1) program was developed by Victor Duchovni of Morgan
Stanley, who also wrote the initial version of this document.  </p>