Try this. I had the same issue a week ago after I rooted. I tried quite a few different things, but nothing worked. I even un-rooted and I still had issues sending/receiving MMS. I found this piece of info from another forum and it worked great! All my MMS incoming/outgoing messages work just fine now.
Hopefully it works for you too!
Note: You can
ONLY do this if you're rooted!
"I was unable to send picture messages after flashing Synergy ROM on an HTC Evo. I did a full wipe and after trying several different things, this was the only thing that worked.
Use root explorer, and make sure it is set to R/W mode (read/write). Navigate to /data/data/com.android.providers.telephony/databases/telephony.db and delete that file, then restart your phone. After the restart, you should be able to send and receive MMS messages."
Unable to send picture texts (MMS). Culprit: Quick Settings App - Android Forums