[afnog] smtp question

[Sergio Carrilho]

Hi all,
I'm trying to set up a domain using postfix and the user in may domain
are able to exchange e-mail between them, and are able to send e-mail
to the world.
The problem is when e-mail is sent from outside. The user from outside
gets the error:

"Mail Delivery Subsystem
<mailer-daemon at gmail.com> to me
	 More options	  Jul 25 (1 day ago)
This is an automatically generated Delivery Status Notification

Delivery to the following recipient failed permanently:

    username at mydomain

Technical details of permanent failure:
PERM_FAILURE: SMTP Error (state 10): 554 <username at mydomain>:
Relay access denied"

and my users aren't able to receive e-mails from outside!!!

Can someone help me with this problem?

thanks

sergio