Try this out. I was having similar problems with my stock MMS not sending... this fixed my problem
1. Download APN Backup & Restore(Android Market)
2. Click Backup APNs
3. Goto your sdcard/ApnBackupRestore
4. Click the apn file and edit under windows or on your phone.
5. Look for the line that says Production and edit.....
<apn name="Production" numeric="00000" mcc="310" mnc="00" apn="1"
user="firstnamelastname@myboostmobile.sprintpcs.co m"(may varie) server="null" password="null proxy="null"
port="" mmsproxy="" mmsport="" mmsprotocol="2.0"
mmsc="http://mm.myboostmobile.com" type="mms" />
6. Save the file(make sure you save it)
7. Open the APN Backup & Restore program
8. Click delete APNs(Very important)
9. Click Restore APNs
10. Close the program
11. Goto messages>menu>settings>connection settings. Make sure everything is as you edited. You should see Name(Production), MMSC(
http://mm.myboostmobile.com), MMS proxy(not set), MMS port(not set), MMS protocol(WAP 2.0).
12. If everything is just as it is, viola you are all set...