Can't send mms to non iPhone users?

I'm using a gsm iPhone 4 with straight talk. It is originally from AT&T however the AT&T compatible SIM card that straight talk provides doesn't let you change the apn settings or even see the cellular data network menu. So I put in the t-mobile compatible SIM card and changed my apn settings as instructed. Now I can only send mms to iPhone users but I can still text and call non iPhone users. Any changes I should make to my apn settings or any other ideas?

See the instructions at the link.

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