Ok, just to update the forum in case it happens to someone else.
I finally spoke to a knowledgeable cust care rep, and the problem was my 'messaging' settings were still using the Web Client.
T-Mobile seems to have changed their process and I simply changed my Messaging Services option from *Web Client* to my current email address.
Before sending out a test e-mail I also had to change the "Using" option from 'Web Client' to my current email address in the compose email window.
Everything is back to normal now.
Life is that much better now.