[otrs] problem with sending mail

I recently made a series of changes to my network; the ability of OTRS to send email has been fouled up by it. Everything is still running on the same host. However, instead of having a dual-homed host with its own firewall, it is now a single-homed host with an external firewall (done the "right" way). When composing an answer to a ticket, I get the following: "CompIT Solutions Support System * invalid helpdesk@compitsolutions.com (mx)!" It returns me to the answer-composition page, however it also does not include the menu-bar at the top when returning me to the composition page. Any assistance would be MOST appreciated. If there are any logs/etc. required, I will provide them.