sounds like a carrier issue and not a software or hardware problem due to the toast message it popped up. have you checked with their web site to see if there are service issues in your area right now? can you send just plain text messages to the same person or no to?
yes i can send regular messages. im with sprint, i havent checked with them i will right now. this has happened before and someone gave me a fix but i cant remember what it was but it worked. pisses me off
Check your APN settings. Are you on T-Mobile APN or still on Sprint? If your data works, ensure message size in the app settings is high enough to support MMS messages. Also, ensure Google Carrier Services is updated/installed.
Yes, download Google Carrier Services from the store. Not sure when tmobile will fully transition Sprint customers to their services or if they will after the merger. On tmobile, APN is fast-t.mobile.com