Although now a standard feature on the iPhone, the first two iPhones did not come with support for the Multimedia Messaging Service. With MMS enabled, cell phone users can send and receive videos, photos and other types of MMS messages. If you have noticed that your iPhone 3GS, 4 or 4S is unable to receive MMS messages, it is most likely due to MMS messaging being disabled on your device. By going into the iPhone's settings, you can quickly enable MMS messaging on your iPhone.
Tap the "Settings" icon on your iPhone's home screen.
Scroll down the list of iPhone settings and tap the "Messages" button.
Scroll down the list of iPhone messaging settings to the SMS/MMS field.
Tap the "On/Off" button next to MMS Messaging to turn MMS messaging on. Your iPhone can now send and receive MMS messaging.
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