1st step: Does it happen with other email clients and the same remote
server? E.g. do use msmtp, Thunderbird etc against the same server for a

Guess: This looks more like the server / the remote end is shutting the
TLS connection down. As you know the server and the port, connect with
openssl s_client a couple of times (it also supports STARTTLS with EHLO,
though you need to get the exact command from the man page
openssl-s_client(1)) and speak some SMTP. Perhaps you can reproduce the

Guess: Could it be that the remote end drops you when under load? Do you
have access to the remote server logs? If not, your service provider
might be able to look up potential errors in their log.

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