J'ai fait le test tout à l'heure
tcpdump - Service POSTFIX arrêté
12:04:31.236994 IP smtp26.services.sfr.fr.51002 > mon_hostname.smtp: Flags [S], seq 2691854497, win 14600, options [mss 1380,sackOK,TS val 1345446644 ecr 0,nop,wscale 9], length 0
12:04:31.237208 IP mon_hostname.smtp > smtp26.services.sfr.fr.51002: Flags [R.], seq 0, ack 2691854498, win 0, length 0
tcpdump - Service POSTFIX démarré
11:02:46.266938 IP smtp26.services.sfr.fr.45475 > mon_hostname.smtp: Flags [S], seq 3390965650, win 14600, options [mss 1380,sackOK,TS val 3097403968 ecr 0,nop,wscale 9], length 0
11:02:46.267052 IP mon_hostname.smtp > smtp26.services.sfr.fr.45475: Flags [S.], seq 3586441830, ack 3390965651, win 28960, options [mss 1460,sackOK,TS val 13798731 ecr 3097403968,nop,wscale 7], length 0
11:02:46.274025 IP smtp26.services.sfr.fr.45475 > mon_hostname.smtp: Flags [.], ack 1, win 29, options [nop,nop,TS val 3097403975 ecr 13798731], length 0
11:02:46.423179 IP mon_hostname.smtp > smtp26.services.sfr.fr.45475: Flags [P.], seq 1:54, ack 1, win 227, options [nop,nop,TS val 13798747 ecr 3097403975], length 53: SMTP: 220 mon_hostname.mon_domaine.fr ESMTP Postfix (Debian/GNU)
11:02:46.430524 IP smtp26.services.sfr.fr.45475 > mon_hostname.smtp: Flags [.], ack 54, win 29, options [nop,nop,TS val 3097404131 ecr 13798747], length 0
11:02:46.430638 IP smtp26.services.sfr.fr.45475 > mon_hostname.smtp: Flags [P.], seq 1:30, ack 54, win 29, options [nop,nop,TS val 3097404131 ecr 13798747], length 29: SMTP: EHLO smtp26.services.sfr.fr
11:02:46.430695 IP mon_hostname.smtp > smtp26.services.sfr.fr.45475: Flags [.], ack 30, win 227, options [nop,nop,TS val 13798748 ecr 3097404131], length 0
11:02:46.431124 IP mon_hostname.smtp > smtp26.services.sfr.fr.45475: Flags [P.], seq 54:188, ack 30, win 227, options [nop,nop,TS val 13798748 ecr 3097404131], length 134: SMTP: 250-mon_hostname.mon_domaine.fr
11:02:46.440042 IP smtp26.services.sfr.fr.45475 > mon_hostname.smtp: Flags [P.], seq 30:40, ack 188, win 31, options [nop,nop,TS val 3097404141 ecr 13798748], length 10: SMTP: STARTTLS
11:02:46.440437 IP mon_hostname.smtp > smtp26.services.sfr.fr.45475: Flags [P.], seq 188:218, ack 40, win 227, options [nop,nop,TS val 13798749 ecr 3097404141], length 30: SMTP: 220 2.0.0 Ready to start TLS
11:02:46.449688 IP smtp26.services.sfr.fr.45475 > mon_hostname.smtp: Flags [P.], seq 40:169, ack 218, win 31, options [nop,nop,TS val 3097404151 ecr 13798749], length 129: SMTP
11:02:46.453097 IP mon_hostname.smtp > smtp26.services.sfr.fr.45475: Flags [.], seq 218:1586, ack 169, win 235, options [nop,nop,TS val 13798750 ecr 3097404151], length 1368: SMTP
11:02:46.453126 IP mon_hostname.smtp > smtp26.services.sfr.fr.45475: Flags [.], seq 1586:2954, ack 169, win 235, options [nop,nop,TS val 13798750 ecr 3097404151], length 1368: SMTP
11:02:46.453133 IP mon_hostname.smtp > smtp26.services.sfr.fr.45475: Flags [P.], seq 2954:3542, ack 169, win 235, options [nop,nop,TS val 13798750 ecr 3097404151], length 588: SMTP
11:02:46.463722 IP smtp26.services.sfr.fr.45475 > mon_hostname.smtp: Flags [.], ack 3542, win 37, options [nop,nop,TS val 3097404164 ecr 13798750], length 0
11:02:46.470144 IP smtp26.services.sfr.fr.45475 > mon_hostname.smtp: Flags [P.], seq 169:303, ack 3542, win 37, options [nop,nop,TS val 3097404171 ecr 13798750], length 134: SMTP
11:02:46.473817 IP mon_hostname.smtp > smtp26.services.sfr.fr.45475: Flags [P.], seq 3542:3776, ack 303, win 243, options [nop,nop,TS val 13798752 ecr 3097404171], length 234: SMTP
11:02:46.482838 IP smtp26.services.sfr.fr.45475 > mon_hostname.smtp: Flags [P.], seq 303:372, ack 3776, win 42, options [nop,nop,TS val 3097404184 ecr 13798752], length 69: SMTP
11:02:46.517361 IP mon_hostname.smtp > smtp26.services.sfr.fr.45475: Flags [.], ack 372, win 243, options [nop,nop,TS val 13798757 ecr 3097404184], length 0
11:02:46.540659 IP mon_hostname.smtp > smtp26.services.sfr.fr.45475: Flags [P.], seq 3776:3973, ack 372, win 243, options [nop,nop,TS val 13798759 ecr 3097404184], length 197: SMTP
11:02:46.550019 IP smtp26.services.sfr.fr.45475 > mon_hostname.smtp: Flags [P.], seq 372:505, ack 3973, win 47, options [nop,nop,TS val 3097404250 ecr 13798759], length 133: SMTP
11:02:46.550158 IP mon_hostname.smtp > smtp26.services.sfr.fr.45475: Flags [.], ack 505, win 252, options [nop,nop,TS val 13798760 ecr 3097404250], length 0
11:02:46.750396 IP mon_hostname.smtp > smtp26.services.sfr.fr.45475: Flags [P.], seq 3973:4122, ack 505, win 252, options [nop,nop,TS val 13798780 ecr 3097404250], length 149: SMTP
On voit bien que ça log encore quand "Postfix" est arrêté, mais beaucoup moins.