I haven't received any calls or other response to the PM mentioned above, but it appears that email messages are once again being transmitted.
If everything continues to work the way I expected it would when I purchased the plans and feature packages, I'll consider the problem resolved.
I'm only one of hundreds or maybe thousands of MMS camera owners who are AT&T customers (at least until cameras that can be used on a CDMA network become available). I had multiple messages deducted from the package balance of three acoounts (over fifty from one of them), but never received them.