[horde] 550 Error: Not allowed

[Mike Poznecki]

I know technically this is not a Horde question, but I'll ask anyways.
I have a new horde server set up.  I have connected it to another server running Dovecot/Postfix.
I can log into my email fine.  But when I try to send myself an email, I always receive a 550 Error: Not allowed.
This is happening between the Postfix server and the Dovecot server.  Does anyone know why?  Am I not sending the right credentials to Postfix?