Re: SMTP error from remote mail server after end of data: 421 Lost incoming connection

Am 21.02.26 um 09:39 schrieb Evgeniy Berdnikov via Exim-users:
> On Fri, Feb 20, 2026 at 04:58:38PM +0100, Elmar Haneke via Exim-users wrote:
>> On Sending mails from local system to external smarthost on some messages I
>> do repeatedly get the error "SMTP error from remote mail server after end of
>> data: 421 Lost incoming connection" on sending local system.
>>
>> The receiving smarthost does write an "SMTP connection lost while reading
>> message data" into mainlog.
>   This is a typical pattern of broken Path MTU Discovery bitween those hosts.
>   Collect a traffic dump on both sides to check for it.


This hint did not resolve the problem.

Setting hosts_avoid_esmtp to enforce simple SMTP on connection to 
external smarthost seems resolve that issue.

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