RRalston

@toddlock Something does seem right from what you have submitted.
You indicated: "Manual email sent from a PC to ##########@tmomail.net works." Doesn't that mean you successfully sent an email which was delivered as a text to your T-Mobile phone?
For you to experience "TEST Button Result is always an immediate “Mail server failure." potentially indicates your router/firewall may be blocking those ports. "Mail Server failure" is different from the carrier blocking the action. I don't think you would get any response like that if you were able to successfully connect to the mail server.
I have xfinity mobile, which is verizon underneath and I send texts to my phone all the time using "<cell.phone.number>@vtext.com
But I also found this in the Wiki related to gmail
Configuring gmail Account
https://wiki.universal-devices.com/ISY994:Configure_Mail_Server_Settings#SMTP_Errors:~:text=Configuring%20gmail%20Account,-%28NOTE
Hope this may help you.
Rob

I have used "${sys.program.#.name}" for that purpose.
But I have also found in many cases, just using the "Default" email content yields a lot of good information.