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...to make a STARTTLS connection to Postfix's submission port. I don't see that in your logs. https://imgur.com/a/3MJXRUh This is the configuration. Afaik thats also the correct port for SMTP. I've also checked that all the ports are open on the server (via netstat): https://pastebin.com/tY6rSi8y I have (currently) no firewall installed so there aren't any ports blocked either. Since it's in an internal network there is also no ISP which can block any ports (and I've also already added a few mailboxes from other sources without any problems). > I still see "warning:...

Postconf: https://pastebin.com/vTVn2UMr Doveconf: https://pastebin.com/nEpZrpzB all my logs from mail.(warn,err,info,log): https://pastebin.com/ccEVUqyd my messges log: https://pastebin.com/cXaEBmcH I'm not sure if you meant this about dict_nis_init but I've done what's explained here: https://unix.stackexchange.com/questions/244199/postfix-mail-logs-keep-showing-nis-domain-not-set