Question : How do I get this e-mail error resolved?

We have 2 servers on the same network, one is an IMail server and the other is an Exchange server. We cannot send mail from a domain on the IMail server to an e-mail on the exchange server, but it works vise versa. We have a barracuda spam firewall, and the reason it gives for the error is this:
Delivery Status:      Deferred
Delivery Detail:        mail for (mydomain) loops back to myself
What should I do?

Answer : How do I get this e-mail error resolved?

it'll be DNS settings on the IMail server. Set the IMail servers DNS settings to be the same as the Exchange server's ones and all should be well.

And if (as you should be) you have the Exchange server using an internal DNS server, make sure that the internal DNS server has the external domains added and A name records for the MX records added, to the internal IP addresses (ie so if your MX for the exchange domain was mail.mydomain.com you have an A NAME record mail.mydomain.com on your internal DNS server that points to the internal LAN IP address of teh exchange server.)

Hope this makes sense!