okay since you think you know it all check out from SAMSUNG THEMSELVES.
"I have run a check on the database for you and I am sorry to inform you that, the phone is designed in such a way that there is no option to send MMS messages using WiFi network as it can be done only through AT&T network."
You can, in fact, only send SMS/MMS over the carrier network. However, it will still work with Wifi on. "Seems like Samsung is telling you that you can't do it over wifi (which is technically true) and not telling you the whole story."
this is already another thread i am copying and pasting from.
Normally, as long as you have mobile data on, if you send an SMS/MMS with wifi on, your phone will temporarily utilize the mobile network to send the message, then switch back to Wifi. It works fine on my AT&T GS3 and this is how it has always worked, to my knowledge.
"The Samsung phone is only designed in such a way that there is no option to send MMS messages using WiFi network as it can be done only through carrier network."