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 -- ## subscription configuration (requires account): ## https://lists.exim.org/mailman3/postorius/lists/exim-users.lists.exim.org/ ## unsubscribe (doesn't require an account): ## [email protected] ## Exim details at https://www.exim.org/ ## Please use the Wiki with this list - https://code.exim.org/exim/wiki/wiki
