I had this problem on my Verizon Galaxy S4 with two different carriers and I was on the phone more times than I can remember trying to fix it....but I finally found someone who was able to fix the problem. This is what fixed mine: Remove the battery WHILE THE PHONE IS STILL ON, leave it out for about 5 minutes, and replace the battery. Turn the phone back on and then try sending a MMS pic message to YOURSELF. Just send the pic message to your own phone number. Once it sends successfully (hopefully it will) try sending to others. This worked for me and I really hope it helps someone else. This issue frustrated me for well over a year. And if it starts messing up again, just do this again. Another VERY IMPORTANT thing to know: If you are using a lower cost cell network (anything other than the big guys like Verizon, AT&T, Sprint, T-Mobile) you HAVE to turn off WiFi in order to send pic MMS messages!!!! And of course, make sure your mobile data is turned on.
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