We tend to avoid the whole SMTP mess and deliver messages to mobiles and pagers via a modem and the provider's TAP gateway. It works quite well with Verizon and AT&T/Cingular, but I've no experience with T-Mobile. It also avoids the whole mess of failing to alert when your monitoring box has a bad NIC, cable, switchport, etc - of course considering that you trade those for problems with a serial port, cable, modem, or phone line... But it gives us a big (and perhaps false) warm fuzzy that our alerting is 'out of band' relative to our upstream Internet connections.
The folks at Avtech have a nice index of TAP gateway numbers at
http://www.avtech.com/Support/TAP/index.htm Hope you find this useful...