I have the same problem, but I did not understand how to solve it.
It is clear that the problem is not in the mail application, but on the server.
There are two servers that are the same set up,
na odnom server - rabotayet
na drugom - net
I checked all configs of dovecot, postfix, rspamd server
NethServer release 7.6.1810 (final)
[Nextcloud] 15.0.5