Trouble with immature firewalls and postfix.
I run into trouble with one postfix server. This server is located behind a firewall and I got the error postfix/smtpd: lost connection after DATA from … in the mail logs. Debugging postfix with debug <>IP-address>> shows nothing unusually. So it must be another reason for that. Searching around the first match is a problem with MTU and the absense of ICMP between the firewall and the postfix-server found on Heinlein Blog. But this was not the solution. The solution is disabling TCP window scaling. Many thanks to Wietse Venema.
Activation for the moment:
sysctl -w net.ipv4.tcp_window_scaling=0
Make it permanent edit your /etc/sysctl.conf and add these line:
net.ipv4.tcp_window_scaling = 0