Error when sending new MMS to just one person

[salty]

Seems like an isolated incident to the one person. Whenever I try and send her a MMS it gives me an error saying:

Failed to send MMS
The network reported: failure (tr)
Recipient address not resolved

Your MMS was not possible to send.
The reason can either be a temporary error in your carriers network or that your settings are not correct.

The error just started happening today since I was able to send her pictures fine before. I don't believe it's the network since I'm able to send to other people on AT&T from my T-Mobile account and nothing was changed to her contact information or to Swirly MMS's settings. Any idea why this is happening all of a sudden? I'm using version 2.2.4 on a 2g iPhone.

[salty]

Any help anyone?