Qmail needs 54 seconds to relay 100 messages with one recipient.
Default configuration, concurrencyremote = 100.
spike_4% /usr/bin/time ./smtp-source -s 5 -m 100 fist
43.77 real 0.05 user 0.23 sys
Jun 8 12:08:36 fist qmail: 865786116.744366 new msg 39901
Jun 8 12:09:32 fist qmail: 865786172.791473 end msg 39921
Total time: 56 sec
/usr/bin/time ./smtp-source -s 10 -m 100 fist
26.66 real 0.06 user 0.26 sys
Jun 8 12:06:20 fist qmail: 865785980.185885 new msg 39901
Jun 8 12:07:14 fist qmail: 865786034.306429 end msg 39920
Total time: 54 sec
spike_8% /usr/bin/time ./smtp-source -s 20 -m 100 fist
20.94 real 0.11 user 0.27 sys
Jun 8 12:10:52 fist qmail: 865786252.412648 new msg 39901
Jun 8 12:11:46 fist qmail: 865786306.080605 end msg 39962
Total time: 54 sec