messages from email gateway connections refused

I'm trying to set up the SMS framework using the email gateway. When I try to send out a confirmation message, I get the following error in the postfix log:

host mx.cingularme.com[209.183.32.63] refused to talk to me: 421 Service not available

I'm running this as localhost using a relayhost of outbound.mailhop.org (through dynamic dns), and postfix seems to be working fine to send email to other email accounts, but it seems to be blocked by AT&T's mailserver. Is there a way to fix this?