How to fix mms on iphone 4 tmobile 6.1?

I upgraded my iphone 4 that i have unlocked for tmobile to the new ios 6.1 but when i send an mms to a person who doesn't have an iphone or imessage, it doesn't send.anybody know the solution?

You would need to jailbreak and install the tmobile mms fix from cydia.

Use reply 106 (linked below). Make sure cellular data is off so you can enter everything into the "cellular data network" section, and make sure 3G and Wi-Fi are off, too. It might take a few tries, and definitely make sure to go to home screen then turn your phone off and on.It'll take a few minutes to register but it worked for me, so I hope it works for you, too.

I had the exact same problem, wouldn't have even upgraded to 6.1 if I knew it would be this much trouble. No worries though, I did the following and once again and able to send and receive mms messages…

Go to settings>general>cellular>cellular data network

then go down to MMS and type the following:
APN: internet2.voicestream.com
MMSC: http://mms.msg.eng.t-mobile.com/mms/wapenc
MMS Max Message Size: 1048576
MMS UA Prof URL: http://www.apple.com/...uaprof.rdf

turn your phone off, then turn it back on. Your mms messaging should not be available and working.