From: Wietse Venema (wietseporcupine.org)
Date: Tue May 07 2013 - 11:57:27 CDT

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Viktor Dukhovni:
> That said, clearly some packets from the sender are lost, and never
> retransmitted. The TCP connection negotiates selective ACK and
> window scaling on both sides. First thing I would do is disable
> window scaling on your Postfix server. This will reduce throughput
> for mail from far away senders (without window scaling Hong Kong
> to USA will be at most 64KB per ~0.25 sec or 4 sec per MB of data).

Postfix 2.6 and later have a workaround for TCP window scaling bugs.
For detailed instructions, see:
http://www.postfix.org/postconf.5.html#tcp_windowsize

        Wietse

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