"Undeliverable" on Nextel text message

I'm wondering if anyone out there has had this happen to them and if they found a solution. We are running MS SBS (Exchange) 2003.
We use Nextel phones (that are actually on the Nextel network - not Sprint PCS) to send appointments to our service technicians. We use Automated Services (by Ascent) which is an add-on to MAS 90. The appointment is typed up in AS and then "paged" to the technician - AS opens MS Outlook and sends it as an email with the recipient's address in the phonenumber@messaging.nextel.com format. On Tuesday, Nextel's website went down, and our technicians have since stopped recieving these messages. The messages are being returned to us as "Undeliverable" after they time-out in the queue. We can email to other domains through Outlook; we have tried sending test messages from other email addresses to the phones and it's hit and miss as to whether it works (gMail, Yahoo work, Juno and our IT company's domain - which is also running Exchange do not). We can send messages from the Nextel website and the phones will receive them, so Nextel denies any responsibility for the problem. I have a call in to our IT company to check the reverse DNS, but other than that I'm pulling my hair out. Please help!