MX needs 12 seconds per 1000 different remote destinations.
smtp process limit = 100, bundle_recipients = 0.

/usr/bin/time ./smtp-source -r 1000 fist
        1.13 real         0.07 user         0.27 sys
Jun  8 13:32:18 fist mx:smtpd[18174]: connect from spike.porcupine.org(168.100.1
Jun  8 13:32:31 fist mx:smtp[18209]: 085688: to=<544foo@spike.porcupine.org>, re
Total time: 13 seconds

/usr/bin/time ./smtp-source -r 2000 fist
        2.55 real         0.21 user         0.48 sys
Jun  8 13:33:23 fist mx:smtpd[18174]: connect from spike.porcupine.org(168.100.1
Jun  8 13:33:48 fist mx:smtp[18184]: 085693: to=<1041foo@spike.porcupine.org>, r
Total time: 25 seconds

/usr/bin/time ./smtp-source -r 5000 fist
[test generating machine ran out of resources while receiving mail]

/usr/bin/time ./smtp-source -r 1000 fist
        1.38 real         0.17 user         0.16 sys
Jun  8 15:20:33 fist mx:smtpd[27695]: connect from spike.porcupine.org(168.100.1
Jun  8 15:20:46 fist mx:smtp[27724]: 085687: to=<493foo@spike.porcupine.org>, re
Total time: 13 seconds

/usr/bin/time ./smtp-source -r 2000 fist
        2.64 real         0.23 user         0.46 sys
Jun  8 15:20:52 fist mx:smtpd[27695]: connect from spike.porcupine.org(168.100.1
Jun  8 15:21:16 fist mx:smtp[27743]: 085687: to=<1086foo@spike.porcupine.org>, r
Total time: 24 seconds

/usr/bin/time ./smtp-source -r 5000 fist
[test generating machine ran out of resources while receiving mail]