Qmail needs 59 seconds to deliver 100 SMTP messages to one local recipient.
Default configuration, concurrencyremote = 100.
/usr/bin/time ./smtp-source -s 5 -m 100 -t wietse fist
41.45 real 0.07 user 0.21 sys
Jun 8 12:17:20 fist qmail: 865786640.494072 new msg 39901
Jun 8 12:18:22 fist qmail: 865786702.301087 end msg 39982
Total time: 62 sec
/usr/bin/time ./smtp-source -s 10 -m 100 -t wietse fist
26.50 real 0.10 user 0.22 sys
Jun 8 12:14:49 fist qmail: 865786489.089492 new msg 39901
Jun 8 12:15:48 fist qmail: 865786548.316898 end msg 39928
Total time: 59 sec
/usr/bin/time ./smtp-source -s 20 -m 100 -t wietse fist
21.15 real 0.08 user 0.30 sys
Jun 8 12:19:18 fist qmail: 865786758.939755 new msg 39903
Jun 8 12:20:16 fist qmail: 865786816.739912 end msg 40031
Total time: 58 sec