Not sure if you got this resolved. I am new to Tmobile
I had a problem similar to yours I would get an error when I tried to setup the email on the phone through the setup and then even through tmobile my.tmobile.com site.
What it turned out was that the PIN # on my phone was not released from the system from my old provider because I switched from Dobson/CellularOne over to ATT. So it was still tied to their network.
When I signed up with TMOBILE I had to give them my PIN# for the phone and the IMEI number and it became embedded in the account. So when I tried to start the email thing it got stuck. So when I called ATT ( they bought out Dobson/CellularOne ) they was able to go into the system and delete it out. Then I went back to the mytmobile.com site and logged in and went to set up the email thing and it popped up with my pin and wanted asked me to add email address and setup a TMO one. So that is what I think the problem is.