Hey guys I had a problem like this. Straighttalk sim. Att samsung captivate. Could receive but couldn't send MMS. Data enabled and APN settings correct. Internet worked. Everything except sending MMS. What I didwas for a ccombination of working APN settings from trial and error. I'm using the newest slimbean on sghi897
First go to your settings.
Go to mobile networks... Network operator. Let it search until you can manually select. If you can select ATT and the registration is successfull then you can use these settings
Name: straighttalkATT
APN: WAP.tracfone
Port: 80
Mmsc:
http://mmsc.cingular.com
MMS proxy: proxy.mvno.tracfone.com
MMS port: 80
MCC: 310
MNC: 410
APN type: default,supl,MMS (no spaces after commas!)
Hopefully that works for someone having the problem I was. Its tricky and I just kinda figured it out so I wanted to share somehow
P.S. manually delete all other APN setting by clicking them as if you were to edit, press your menu hard key, and select delete. Gone
PPS... I have a T-Mobile sim card hence the combo of att and t mobile APN settings. Used T-Mobile for internet and att mms