Yes, I guess this is true, just because it is very old.
But this is running at background very often, unreachable from outside, but then maybe blocked from very motivated IT security. :)
In my case it is the Nextcloud webmail client and I want to share how to change the Message-Id Header to prevent this!
Go to the file:
<NEXTCLOUDPATH>/apps/mail/vendor/bytestream/horde-mime/lib/Horde/Mime/Headers/MessageId.php
to line 35 and change it to e.g.:
Best...