Contacts re email deliverability problem to

That may be, but it would surprise me. The carriers still get paid
by virtue of charging the recipients for the SMSes, and in this
particular case cutting off this line of communication is leaving
money on the table, as email->SMS deliverability is desired yet
optional/secondary functionality of the app.

Paying to *receive* SMS? And I thought the UK mobile industry was doing a good job of screwing its customers...

In all seriousness, I wonder if they're using the same platform for their email gateways world-wide, and / or rate-limiting globally on the assumption that they're *not* making any money, as would be the case in many other locations? Especially if the Tmobile US operation is tech-driven (or otherwise) from the German parent.