fwiw, another one of those "can't quite get there from here" scenarios;
Cingular had two internet to text message domains which allowed email to
be sent from any email client to a cingular cell phone;
<cellnumber>@CingularME.com
and
<cellnumber>@mobile.mycingular.com
The acquisition by at&t added a third;
<cellnumber>@txt.att.net
While all of the above work (at the moment) for someone to send text
_to_ your phone, their digital coded "reply" addresses are confusing,
broken or severely delayed, plus the two cingular addresses have changed
behavior since I started using them.
The @CingularME.com address includes a binary "reply to" number which is
accepted without argument by the system if you send a reply... however,
it appears to "black hole" the relies... sent a couple of test replies
to messages from this address earlier this morning and they have yet to
arrive.
The "36245" sender address coming with mail sent via the last two
addresses above is at&t's 5 digit code for "Mail" ("6245" is "Mail" on
the number pad) ...if you simply send a reply to that number, it will fail.
To use "36245" return address, you must manually put the "to" email
address(es) in the reply message's subject line but that's not clear to
the user in the original message or on AT&T's web site.
Now for the final kicker... if the received message is from someone who
used the @txt.att.net address, the sender's email address is not
included in the text message.
so, for the moment, your internet text mail correspondents need to use
the @CingularME.com address as that includes the email address ...unless
you know their email address by heart.
Hope this helps someone else.
Beverly Howard [MS MVP-Mobile Devices]