I'm certainly no professional when it comes to phone dev and this may be something that is already widely known, but I know it is something that I struggled with when I first started fiddling with my Note 3...
I found that with some of the ROMs I was flashing that my MMS wouldn't work with Hover Chat (That's the sms app that I use because I'm slightly small minded and have an affinity for chat bubbles). Anyway, I tried flashing some zips I found here and elsewhere but nothing worked. In the end I found that simply putting http://mms.sprintpcs.com
into the MMSC field worked perfectly for me.
I don't know if some of you actual developers know something about that that might be wrong or if there is a better fix... Thoughts?
"Don't believe everything you think"