Re: [exim] Google SMTP Timeouts on large mails

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Author: Adam D. Barratt
Date:  
To: exim-users
Subject: Re: [exim] Google SMTP Timeouts on large mails
On Fri, 2022-04-29 at 10:56 +0100, Graeme Coates via Exim-users wrote:
> Hi all,
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> I've seen this issue raised in:
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> https://lists.exim.org/lurker/message/20220216.071725.892984cd.en.html
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> and
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> https://lists.exim.org/lurker/message/20220313.200645.624cc373.en.html
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> but haven't seen a definite resolution as yet.
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> As per other reports, I have a Debian Bullseye (11.3) system running
>


This is likely to be the result of a known issue with Google's TCP Fast
Open setup - see e.g.
https://blog.apnic.net/2021/07/05/tcp-fast-open-not-so-fast/

Exim 4.93 changed the default for the "hosts_try_fastopen" transport
option to be "*", and the default for the
net.ipv4.tcp_fastopen_blackhole_timeout_sec sysctl changed from 3600
(i.e. an hour) to 0 at some point between the kernel versions in Debian
buster (10) and bullseye (11).

A workaround is to add something similar to "hosts_try_fastopen = !
*.l.google.com" to your SMTP transports.

Regards,

Adam