I don't know if it's a problem that affects Windows Mobile 5.0 Smartphone Edition or only the I-Mate SP5m device, but here's the facts:
I had a big problem on receiving and sending MMS with my new I-Mate SP5m. I was sure to have set the phone correctly (my GPRS works good) and I've also downloaded the configuration CAB for my operator from the I-Mate site and installed it... Result? MMS didn't work. Seems tha they tryed to start but they're not able to be sent or received.
After lots of trouble, a doubt comes on my mind: maybe a memory trouble?
I've start to uninstall some unuseful preinstalled applications (like the Backgammon game) and others thay I've previously installed on the phone memory and after this process I've rset the phone (turn it off and on).
Next, I've tryed to send an MMS... Result? It was sent!!!! 
The result of this was the fact that with Windows Mobile 5.0 Smartphone Edition and the I-Mate SP5m, if you don't have at least 4Mb of free RAM, you're not able to send or receive MMS. I don't know if it's a Windows Mobile problem or only a device problem, but this is the fact.
When you install new applications, use the SD card...