Help with mail server. "Recipient address rejected: No such domain." (Behind router)

[schlongsalami]

I have set up vPostMaster (which is just like postfix i believe). Also I have installed SquirrelMail.

I can perfectly receive emails but not send them other than to local users.
When I try to send an email to pblach@gmail.com for example, I get this error:

Transaction failed
Server replied: 554 <pblach@gmail.com>: Recipient address rejected: No such domain.

I belive this has something to do with the fact, that the server is behind a router.
I also believe, it is not the port forwarding, but rather the mail server configuration. (See below for the forwarded ports.)

Does anybody know how to fix this problem? I am absolutely lost and can't find the answer on google either.

Thank you.

PS: Here is the list of ports that I have forwarded in the router:
Port 1
Protocol: TCP
Global Port(s): 110
Base Host Port: 110

Port 2
Protocol: TCP
Global Port(s): 143
Base Host Port: 143

Port 3
Protocol: TCP
Global Port(s): 993
Base Host Port: 993

Port 4
Protocol: TCP
Global Port(s): 995
Base Host Port: 995

Port 5
Protocol: TCP
Global Port(s): 25
Base Host Port: 25