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Postfix and lost connection after DATA

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[1434]: 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