Thank you for this thread! I have been searching for a working solution to help a friend get the stock messaging app to work on his Verizon S4 flashed to Boost (no SIM card). He can send/receive SMS and can receive MMS. But can't send MMS. Yesterday, I decompiled the stock messaging app to see if I can find anything that sticks out...nada...but I have to admit, I only spent an hour looking.
Have this same problem as well on a Verizon Note 3 and Note 2.
I agree that Go SMS Pro is not an ideal workaround. It's too damn bloated.
I experimented with the following mmsc in /system/etc/apns-config.xml to see if the phone's MDN can be sort of hard coded in, but unfortunately, it didn't work. I've read that this workaround worked fine on other phones (older phones that is) so it was worth a try.
mmsc="http://mm.myboostmobile.com/servlets/mms?X-SPRINT-MDN=3105551234", where 3105551234 is your 10 digit MDN/phone number.
Flash Galaxy Note 3, Note 2, S4, S3, S2 to Sprint, SERO, Boost, Virgin Mobile: CLICK HERE
MMS Fix for Galaxy Note 3, Note 2, S4, S3, S2 on Sprint, SERO, Boost, Virgin Mobile: CLICK HERE