I've come across this one before and it can be one of two issues and is more to do with strrict RFC compliance than a technical issue persay.
Issue 1For example if the Endian is doing an MX lookup and it's getting it's own IP address back it will bounce email back to the sender as sending back to yourself.
Check your internal DNS servers are reporting the destination MX record correctly.
Issue 2The next fix is more common, in your MS Exchange SMTP connector under delivery and then advanced, make sure that the Fully-qualified domain name box has a different value in it than the one your Endian is using.
Ie if Endian tries to send email to your Exchange and it greets it with the same EHLO name then it will assume it's the same box and not send it.
Hope this helps