You could try the faster 3g script posted in the rooted section "[SCRIPT] rtsp & httpd proxy settings (faster 3g)" and see if that works. Other than that I dunno what else to do, other than checking all settings but you probably have checked that already.
I had the same problem on 2 separate phones. This fix worked both times. Use a file explorer with SU, and 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