Stop what? An MMS is a text message with a link to a picture, video or sound file. If the file is small enough it may be sent as part of the text message, but almost all multimedia files will be too large, so you'll get a link to the file (which the carrier is storing on a temporary server location for you).
If the message itself is sent as a text file, or it's longer than 160 characters (that's due to the specifications of SMS messages - nothing to do with the phone or the carrier), the same thing happens - a link to the message.