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From: Viktor Dukhovni (postfix-usersdukhovni.org)
Date: Tue May 07 2013 - 09:58:06 CDT
On Tue, May 07, 2013 at 04:46:35AM -0700, mailtime wrote:
> Thanks for your advice Viktor
> I have done a capture and loaded into WireShark.
The wireshark output, which omits much detail, (but the GUI allows
you to drill in various details) is for you to interpret. If you
want help from me, post complete output from "tcpdump -nr capture.pcap".
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).
The problem is almost certainly a firewall bug either at the sending
or receiving side. Updating a more recent firewall software or
firmware version may help.