This is going to sound crazy, but I have tested it in exchanging MMS messages with other iPhones on T-Mobile, Verizon and AT&T and also with Email accounts
1. Backup your iPhone 4S in iTunes
2. Download, install and open iBackupBot (Google it)
3. Select your recent iPhone backup
4. Find and open the file named "com.apple.mms_override.plist"
5. Delete the text in this file and replace it with
6. Save the changes
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
7. Exit iBackupBot and open iTunes
8. Restore your iPhone with the most recent backup
9. After your iPhone reboots go to Settings>General>Network and turn on Data Roaming
10. While still in Network settings go to >Cellular Data Network and enter "epc.tmobile.com" as the APN for Cellular Data and Internet Tethering
11. Enter the following for MMS settings:
MMS Proxy: 126.96.36.199:8080
NOTE: Leave all other fields empty, if they have anything in them, delete it.
I have no clue why wap.cingular works for T-Mobile customers, nor do I care. But as mentioned before, I have exchanged MMS with other phones and email accounts.