Postfix DSN support implementation notes
========================================

In delivery status reports, Postfix now properly reports remote
LMTP/SMTP server replies with Diagnostic-Type: SMTP, with the
Diagnostic-Code: equal to the server reply, and with Remote-MTA:
equal to the name of the remote MTA.

Of course Postfix still produces the same "informal" error descriptions
that it produced before (for example, the error text that appears
in the first section of a bounce report).

Other error reports are not in the form of SMTP-style replies.

- The Postfix LMTP/SMTP client generates Diagnostic-Type: X-Postfix
for locally generated errors (host not found, connection timed out
etc.).  It generates Diagnostic-Type: SMTP only for replies from
an SMTP server.

- The queue manager generates Diagnostic-Type: X-Postfix for errors
that it detects. It also receives error information from delivery
agents and reports that information unmodified when it decides to
"temporarily suspend" a delivery channel.

- The "pipe to command" code in local(8) and pipe(8) produces
Diagnostic-Type: X-UNIX, and Diagnostic-Code: text that is taken
from /usr/include/sysexits.h or from the command output.

- The code that delivers to mailbox produces Diagnostic-Type:
X-Postfix and Diagnostic-Code: text that is the same good old
Postfix error message that we are already familiar with. Typically
these are errno-style reports about locking a file or appending a
file.