Just in case anyone else is wondering how to send an SMS text message to their AT&T wireless phone via email, the address apparently has changed through their recent acquisitions. Googling around only revealed the prior (non-working) domains, so I had to resort to the AT&T support page.
Horribly worded and a bit confusing as to which category you fall under, if you’ve signed up to “the new at&t” (as opposed to the prior AT&T, Cingular, etc.), your address is 10digitphonenumber@txt.att.net.
Happy texting!