As soon as I enable my ispconfig firewall I get timeouts in postfix.. I have to disable the firewall and restart postfix to get it working again.
Is that a timeout on incoming (to your server) or outgoing (from your server)? If incoming, what port(s)? (ie. port 25 from external servers, or your own connections on 465/587?) What ports do you allow in the firewall? What does your firewall contain ('iptables -L -n -v') when it is enabled?
iptables -L -n -v https://pastebin.com/jC0Asdj3 I just create this server with this guide: https://www.howtoforge.com/perfect-server-debian-10-nginx-bind-dovecot-ispconfig-3.1/
Ok, so the firewall is unrelated and just reload postfix temporarily clears the issue. Likely your mail log will have indications of what's going on. There are some things you can try listed under http://www.postfix.org/STRESS_README.html in particular, I'd look at setting up postscreen. But you may well have something "broken" in the mail system too, or even just need to throw some more memory/resources at it.
