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

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.

Both machines are debian trixie and have "exim4-daemon-light 
4.99.1-1~bpo13+1" installed. Error also did happen with 4.98.2-1 package.

I guess that something within the mails does make exim fail - but no 
idea what it might be.

What can be done to resolve the problem.


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