Postfix suddenly stopped working

[Petr Janda]

I use Postfix on my mailserver, and all of sudden(no change in configs, 
no software updates done) it stopped working.

Thunderbird says that it is Connected to the server for about 2 minutes 
and then connection failed. Output of tcpdump shows this:

porncatalog# tcpdump -vvv port 25
tcpdump: listening on fxp0, link-type EN10MB (Ethernet), capture size 96 
bytes
04:56:29.066370 IP (tos 0x0, ttl  63, id 37069, offset 0, flags [none], 
length: 64) 192.168.1.1.1920 > porncatalog.smtp: S [tcp sum ok] 
4044775792:4044775792(0) win 57344 <mss 1460,nop,wscale 
0,nop,nop,sackOK,nop,nop,timestamp 4465114 0>
04:56:29.066494 IP (tos 0x0, ttl  64, id 843, offset 0, flags [none], 
length: 64) porncatalog.smtp > 192.168.1.1.1920: S [tcp sum ok] 
1108842486:1108842486(0) ack 4044775793 win 57344 <mss 1460,nop,wscale 
0,nop,nop,timestamp 70743 4465114,nop,nop,sackOK>
04:56:29.067446 IP (tos 0x0, ttl  63, id 37070, offset 0, flags [none], 
length: 52) 192.168.1.1.1920 > porncatalog.smtp: . [tcp sum ok] 1:1(0) 
ack 1 win 57920 <nop,nop,timestamp 4465114 70743>

It seems that postfix fails to respond for some very strange reason. Can 
someone give me some clues? I tried restarting server, makes no 
difference. It only happens with Postfix, all my other services work 
correctly. Running DF 1.6-RELEASE.