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

Evgeniy Berdnikov via Exim-users <[email protected]> Sat, 21 Feb 2026 11:39:44 +0300

Newsgroups	gmane.mail.exim.user
Message-ID	<[email protected]>

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.
-- 
 Eugene Berdnikov

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