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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