[DrSuSE]

As far as I know, Sprint does not provide their own SMTP service. Additionally, no one (ATT) is going to let you send through their SMTP server without authentication. You need to find out if they offer that, SMTP from outside networks via authentication. If so, you should be fine using that. If not, you'll need to find an SMTP server/service you can use (it doesn't need to be tied in any way to the POP3 account).