Please DO NOT QUOTE this post in your reply. Just post your questions or comments if you have any. I DON'T CHECK OR RESPOND TO PM'S.
5/31/2014 Update - The Palm Treo Pro 850 and HTC Touch Pro's MMS are now working again as well.
5/20/2014 Update - As of 5/19/2014, Sprint has removed the HTC Touch Pro2 from the list of decommissioned picture mail phones. MMS is now working again on the HTC Touch Pro2.
5/1/2015 Update - As of 4/30/2014, if you have a flashed phone, Sprint has decommissioned a bunch of older phones as far as MMS functionality is concerned. For more info, look here: https://community.sprint.com/baw/co.../2014/01/31/picture-mail-decommissioning-faqs and https://community.sprint.com/baw/thread/152977?start=0&tstart=0. This affects lots of donor phones including, but not limited to HTC Diamond, HTC Touch Pro, HTC Touch Pro2, Palm Treo 850 Pro, etc. The only way to get your MMS back on your flashed phone is to replace your donor. For Sprint legacy plans such as SERO, the Sprint Sanyo Incognito still works. For Boost, the Boost Sanyo Incognito works as well. Boost phones can't be activated with Sprint, but select Sprint phones can be activated with Boost. None of the mentioned methods below will fix your MMS if your donor phone's MMS has been decommissioned by Sprint.
Background: I bought 2 Galaxy S3's. One had the problem out of the box, and the other was just fine. A friend had a problem as well. It seems some Galaxy S3's may have shipped with a defective APN config setting. Method 1 and Method 3 are the "real" fixes, but they require a rooted phone. Method 2 is more of a work around and doesn't require a rooted phone. You must use 8sms app instead of the stock messaging app for SMS/MMS messages.
If you'd like to thank me, please click on the "Thanks" button instead of posting a thank you note. Let's save storage and bandwidth. And please, please, please...DO NOT QUOTE THIS POST IN YOUR REPLY JUST TO THANK ME! 
Method 1 - (the real fix, but requires a rooted phone):
***UPDATE***
I've confirmed that this method works on ICS (4.0.4), Jellybean (4.1.1, 4.1.2, 4.2.2, 4.3) , and KitKat ( 4.4.2) . Tested and working on Galaxy S3, S4, Note 2, and Note 3.
1a) Download this working Sprint APN xml file:
https://www.box.com/s/40x5ac2c6u0ckzfz59ig (click on download button at the top)
OR
1b) For Boost, download this xml file:
https://www.box.com/s/o9folfrq00ee9p2qj5pv (click on download button at the top)
OR
1c) For Virgin Mobile, download this xml file:
https://www.box.com/s/9itl21drpp84enqeszc9 (click on download button at the top)
2) Download and install "APN Backup & Restore" from the Google Play Store to your phone:
https://play.google.com/store/apps/details?id=com.riteshsahu.APNBackupRestore&hl=en
3) Convert the "APN Backup & Restore" into a system app by either using Titanium Backup, or simply use Root Explorer/ES File Explorer (root enable it) to move the APK from /data/app to /system/app. For KitKat, move it to /system/priv-app.
4) Reboot the phone.
5) Launch the app, click on menu, then click on "Disable ICS Check".
6) In the main app menu, click on "Backup APNs". This will create a directory called "ApnBackupRestore" in your /mnt/sdcard.
7) Copy the xml file from step 1 to the /mnt/sdcard/ApnBackupRestore folder.
8) Go back to the main app menu, click on "Delete APNs" and confirm it.
9) Go back to the main app menu, click on "Restore APNs".
10) Choose the file you downloaded from step 1 and confirm it.
11) Reboot your phone.
12) Done.
NEW Method 2 (work around, doesn't require rooted phone) - Uses 8sms app which is lightweight unlike other 3rd party bloated messaging apps such as Go SMS Pro, Handcent, etc. More details about the extra features of this app can be found here:
http://xdaforums.com/showthread.php?t=2398767
1) Download and install 8sms app to your phone from the Google Play Store
here or from the developer's site
here.
2) Launch 8sms app.
3) Press Menu button on your phone.
4) Choose Settings
5) Choose MMSC Settings
6) Press "MMSC".
7a) Type in "http://mms.sprintpcs.com" - do not include the double quotes. Press OK button.
OR
7b) Type in "http://mm.myboostmobile.com" for Boost
OR
7c) Type in "http://mmsc.vmobl.com:8088/mms?" for Virgin Mobile
8) Press "MMS PROXY".
9) Type in "68.28.31.7" (Sprint & Boost) OR "205.239.233.136" (Virgin Mobile) - do not include the double quotes. Press OK button.
10) Press "MMS PORT".
11) Type in "80" (Sprint & Boost) OR "81" (Virgin Mobile) - do not include the double quotes. Press OK button.
12) (optional) Press "MMS over WiFi". I know this works for Sprint. Not sure if it works for Boost or Virgin Mobile.
13) Press phone's back button.
14) Choose "Phone Number" under the Extra Settings section and enter your 10 digit phone number.
15) Press OK button.
16) Press phone's back button.
17) Send/receive a few SMS/MMS messages to test.
18) (optional) You can also configure the app's extra settings such as enabling EMOJI, etc.
19) Done.
Hidden below is the OLD Method 2 - Uses Go SMS Pro which is too bloated and doesn't work well with apps such as Avast Mobile Security
Method 3 - (also a "real fix", but requires a rooted phone and custom recovery such as TWRP, ClockWorkMod, etc.) PLEASE LEAVE FEEDBACK IF THIS METHOD WORKED FOR YOU!
Disclaimer: I'm not responsible if this breaks your phone. Proceed at your own risk! These scripts will replace your old apns-conf.xml file in /system/etc and will delete your telephony.db. When your phone reboots, the telephony.db will get recreated according to the content of your apns-conf.xml file. Thanks to oakieville for the script!
1) Boot into your custom recovery.
2) (optional, but highly recommended) Perform a nandroid backup using your custom recovery before you apply this fix just in case!
3) Not sure if this is necessary, but go ahead and wipe cache and dalvik cache.
4) Flash ONE of these:
Sprint (original, stock, working MMS for Sprint):
https://app.box.com/s/m2pq7sesi6aexwa9zu3g
Boost:
https://app.box.com/s/dt16cd44hsix3n2dl0mf
Virgin Mobile:
https://app.box.com/s/395osivovclbbr44p5w1
5) Reboot your phone.
6) Done.
7)
Please leave feedback if this method worked for you!!!
If you'd like to thank me, please click on the "Thanks" button instead of posting a thank you note. Let's save storage and bandwidth. And please, please, please...DO NOT QUOTE THIS POST IN YOUR REPLY JUST TO THANK ME! 
