If you're talking about what email address you can use to send an SMS to your phone, it is <your number>@txt.att.net for me, or if that doesn't work then one of these: http://en.wikipedia.org/wiki/SMS_gateways
mms doesn't work on the iPhone. Ancient technology. iPhone uses email, so take a picture and attach it to email and send. If someone sends you an MMS, there is an MMS website at&t sends you to in order to view the tiny fuzzy mms picture which is: http://www.viewmymessage.com/en/legacy/login.jsp