I am experiencing the same issue on one of my servers.
All domain settings are green in Mail Server App.
I've tried changind permission without any success:
chmod 644 /www/server/dkim/xx.com/default.private
systemctl restart rspamd
I've also ran:
openssl s_client -connect mail.135mailtest.com:587 -starttls smtp
return code is 0 (ok)
Do you have any other suggestions? I am running CentOS 8.5, aapanel is 7.0.7 and Mail app is 5.1.1
P.S. I've also replaced the SSL certificate on mail server.