Hey,
Today I unlocked my AT&T HTC One to transfer to T-Mobile since AT&T has just been awful and deceitful.
Calling, internet, and text messages all work perfectly fine. I just cannot send or receive MMS messages. I set up the APN as it needs to be per Network: HTC One (M8), and have rebooted my phone, but still cannot send or receive MMS.
You can see a screenshot of the APN settings on my phone in these pictures: http://imgur.com/WDegr7G and http://imgur.com/FxW2LBa
Thank you