Rob A:
> Wietse,
>
> Thanks for your response.
>
> I can consistently send a 1 MB attachment to a recipient with no issues, but
> consistently get "421 4.4.2 service timed out. (in reply to end of DATA
> command)" with a 3 MB attachment sent to the same recipient.
>
> I have turned off tcp_window_scaling on my postfix server:
>
> *# cat /proc/sys/net/ipv4/tcp_window_scaling
> 0*
>
> And tcpdump indicates no scaling is used as well:
>
> *[Window size scaling factor: -2 (no window scaling used)]*
>
> Is there something else I should be looking at to resolve the issue?
Yes. IP path MTU discovery. It breaks when some firewall is dropping
ICMP messages indiscriminately.
If totally desperate, you can try to set a smaller MTU at your
end, to overcome some narrow channel elsewhere.
mtu=1500 (normal)
mtu=1476 behind GRE tunnel
mtu=???? Connected via PPPoE
Wietse
