Sorry, Port 25 is also open on the router. I didn't mention it seeing that I have no problems sending E-mails via Squirrelmail.
Also no problems telnetting into port 25 internally:
user@server:~# telnet 192.168.2.60 25
Connected to 192.168.2.60.
Escape character is '^]'.
220 domain.tld ESMTP Postfix (Ubuntu)
I'm all out of options guys
Any other ideas?