Currently there seems to be a bug in the Sophos UTM that prevents a TLS connection from being established.
Im Logfile des MTAs des Absenders finden sich dann Einträge mit der Meldung „TLS Handshake failed“. Hier im Beispiel mal von Clearswift:
There is also a message about this in the Sophos forum:
https://www.astaro.org/local-language-forums/german-forum/54875-bug-9-210-20-smtp-send-receive.html
Apparently today is update day with the admins... There is already version 9.3 on the Sophos FTP server, I can't say whether the TLS errors have been fixed there. But it would be worth a try before some mails get lost.
Update:
The problem has probably been fixed in version 9.3, as there is no direct update from 9.210 yet, the version can be changed via the shell so that the update can be installed manually:
echo ‚ 9.209009‘>/etc/Version