mail stuck in queue

Hi
Having bother trying to get email working and this is the stage i am at now:

I can send mail from my server to anyone@anyotherdomain.
When i send email from a user@mydomain to anotheruser@mydomain the message stays in the queue, similarly when i send from anyone@anotherdomain.

The mail is reaching the server so no dns problems – my problem is that they dont get to the corresponding users inbox and therefore cant be read by the mail client or webmail.
Any help resolving this would be greatfully received.
Thanks