Switching SMTP (Simple Mail Transport Protocol) servers [1] will always cause problems. When we first switched to Postfix [2], I had problems with the email interface for mod_blog [3]. It turned out that postfix set the default permissions of child processes to be restrictive and the patch was to relax the default permissions.
Of course, I forgot about that with the software I wrote [4] for Spring [5].
But it didn't really matter since she rarely, if ever, used the email interface.
Until today.
It took me quite a while to track down the problem and add a one line fix.
[3] https://boston.conman.org/about/