I did some testing. Since I have T-Mobile as well and notice my Battery Notification mail was not working since I read your post and found some sensors were below the threshold of my programs that were suppose to notify me. So I checked my email settings in eisy to make sure there were there and such for the group that is to be notified and everything is as it was suppose to be. Using the default SMTP Setting I performed a test. It failed to send SMS/MMS but did send my email copy. Note that I do put the country code in my number as I talked about above. So I tested it without the country code and it failed again with email copy working.
So I setup a eisy email account off one of my domains, setup the SMTP to use my domain MX Server, put the 1 country code back into my address for SMS/MMS and performed the test again. Both the SMS/MMS and email copy were successful.
This might have to do with either UD in house MX Servers or MX Providers. If providers they may not allow phone numbers or specifically @tmomail.net due to SPAM problems to mobile phones using this.
It's something UD will need to look into..
Cheers!
TRI0N