My wife and I have the same problem. it is a Common issue but not constant. I have 626s, she has Max Pro started off with 2 626s. had the same spontaneous texts, I can send and receive from all but my wife. ???
Can you be clearer? The original post was about MMS, but yours mentions texts, which to most people would mean SMS. Although MMS and SMS are handled by the same apps, they are completely different in how they work, so we need to know unambiguously whether your problem is with MMS, SMS or both.
A problem that's limited to just between your 2 phones is probably something different from the original problem in this thread. So can you also clarify whether you both have the same problem, or only one of you?
With MMS, you will have a problem IF you have a VPN service running. If that is so, then you must turn it off.
just a thought, OP did not mention if VPN is in service.
I recently had trouble getting my spare Galaxy S5 to send MMS, all of the solutions were in the settings of the Textra app. In my case, all of the messages were going out as split up SMS. It was not a 'one click' solution.
forget what, but look at your message app. if the HTC/other phone SMS app is not working, then try to use another MMS like Textra which will, if set up correctly, turn ON data if it should be off.