Mailserver diagnoses can be tricky: I personally find the easiest way to do it still using telnet in combination with the server logs to find out what exactly is wrong.
It is very easy to do:
telnet thehost.com 25
mail from: email@example.com
rcpt to: firstname.lastname@example.org
… type some data …
That is all; it will help to exactly determine where it goes wrong.
The case we had today;
– postfix didn’t send mail ; it gave connection refused
– thoughts; firewall, network, dns etc.
– we tried the above and it worked fine, meaning postfix was the one refusing, not the network/firewall etc
– we fixed the postfix config and all was fine