Just a few thoughts... I have no direct experience with the Allworx system, but I moonlight as a systems administrator...

* Is the box doing the sending on the same IP subnet as your mail server? If not, the mail server may be rejecting the message due to it thinking that the voice mail is actually trying to bounce spam off it (relaying).

* Are you sending on to an internal mail server or external? Either may require SMTP authentication to send messages.

* If you're sending out directly to the recipient's mail server, you *may* be hitting spam filter blocks. This opens another can of worms..

Easiest thing would be to use your internal mail server (if you have one) as a smart host. That way, you can send to an "established" mail server that you know works fine.