421 4.2.0 sender rejected AUP#SNDR

So in preparation for making the switch to Verizon, I stopped by a T-Mobile store to drop off a Sprint Voice Pro device that they sent me awhile back to replace my Airave when my wife and I upgraded our phones (because they weren't compatible with the Airave anymore).

I brought up the issue of the email-to-text not working and the server bouncing back the emails with an error. The guy there confidently stated it was because of the Sprint network integration and said that migrating my account to T-Mobile would fix it. I couldn't remember at the time whether you ( @sdynak ) were already on TMo, but now I see you are when I review this thread.

So apparently the guy was bullshitting. Plus, they wouldn't take back the Voice Pro, but assured me that when I change providers, I'd be sent an RMA kit (though I'm really skeptical I won't get a whopper of a bill for it, given how discombobulated they all seem to be).

I did look at the magenta max plan though and it does appear to be cheaper than what I'm paying now. But if it won't solve this problem or the randomly slow texts or the choppy calls from inside our house even with WiFi calling enabled...