I'm using an ATT branded, black HTC One. My SIM is for T-Mobile. Like many others, I'm experience issues with sending/receiving MMS. I had been using Android Revolution HD 71.1 and would be unable to send MMS. Strangely, though - my ability to send them seemed locations specific. At work it was no problem, at home it would fail every time.
Unless I disabled WiFi. With WiFi disabled, they seemed to send no problem, but with WiFi and Mobile Data simultaneously enabled it would return errors. Disabling WiFi while at home was an inelegant solution, so I tried downgrading to Android Revolution HD 30.6, which is Android 4.3 rather than 4.4.2. This seemed to have done the job. Until it no longer worked.
Same issue. Wifi disabled - fine. Wifi enabled - unable to send MMS. Everything else works fine, phone calls, mobile internet, etc.
This was meant to be a post advising others how to fix this problem. Instead I'm right back where I started.
This is very frustrating. Especially since I thought I had found a solution only for it to fail once again.