you can search for the String "1844671562067968" in the logs to find the mail that is triggering the problem.
Of course, I was looking for this code (also for “input string”, “FairEmail”, etc.) and was found it cited only in this forum.
I have queried also the general syslog /var/log/messages, /var/log/maillog, /var/log/auth.log, and have also tested disabling the firewall altogether.
