I use the mail-to-SMS gateway to text me at different times of the day. For example, to alert me to do something on my to-do list, or to tell me if some long-running program on my PC, such as a backup, is completed.
Often the text comes through as expected. But, quite frequently, I get a message from GMail telling me that the email has been rejected , with a message similar to this:
421 4.1.0 <firstname.lastname@example.org> sender rejected AUP#CDRBL
That email a via a custom domain that uses GMail to handle mail.
The curious thing is that this email (and a few others that are also custom domains with mail handled by GMail) is not really backlisted (as I think CDRBL suggests). Some messages from that account get through. In fact, Google will attempt re-delivery for quite some time if a message is rejected, and very often the mail does eventually get to me (albeit a day or two late).
My guess is that T-Mobile is rejecting messages relayed by one or another Google mail server, but not all of them. When a message is eventually resent via one of the servers T-Mobile seems to like, it gets though.
Meanwhile it sort of makes the mail-to-SMS gateway less than useful, as the idea of text messaging is that it’s fairly immediate.