My sister recently purchased an unlocked Windows Phone specifically the Samsung Focus S. She was using the phone and recently realized that MMS capabilities were not working, she could neither send or receive pictures or video messages. The first thing I check was that she was on the latest version of Mango\Tango. After verifying that I did the following:

  1. Go to the WP Marketplace and tap Samsung Zone
  2. Download the Wireless Manager App
  3. Once the app is fully installed go to the apn screen and tap on 'edit apn'
  4. Tap the '+' at the bottom
  5. Fill out the data as follows: (leave the other fields with their default settings)
    APN Name: mms_line
    Profile Category: mms
    MMSC: http://mms.msg.eng.t-mobile.com/mms/wapenc (ignore any error messages as you fill this out)
    APN: epc.tmobile.com
  6. Click the checkmark button on the toolbar and if prompted overwrite any existing settings.
  7. As you leave the app ensure that 'Automatic APN' is unchecked.

Try to send an MMS message … it should work!