[iZm]

This has been covered several times here. I don't remember who posted it originally, but here are the email domains for each provider as we have it here in the states:
Sprint : MDN@pm.sprint.com
Verizon :MDN@vzwpix.com
Cingular : MDN@mms.mycingular.com
US Cellular : MDN@mms.uscc.net
T-Mobile : MDN@tmomail.net
Nextel : MDN@messaging.nextel.com
Alltel : MDN@message.alltel.com
AT&T : MDN@mms.att.net
Boost Mobile : MDN@myboostmobile.com
Virgin Mobile : MDN@vmobl.com

Much Luv,
iZm