My phone is sending out mms messages to unknown numbers
I sounds like you have managed to install some malware - check the Application settings for those with permission to send MMS messages and disable MMS functions for any that you don't recognise or need to be able to send messages and possibly uninstall them. It is probably worth doing this periodically anyway.

If you go to Applications in the Settings area, (the gear), and select the options, (the pile of 3 dots on the top right on my version), you can select SMS to get a list of all the Apps that can send SMS, and presumably MMS.