@tmomail.net not working

I'm using iCloud mail now; however, this has happened in the past with gmail and other providers. The majority of my current failure messages say "host tmo-west.mx.a.cloudfilter.net[35.85.199.61]
   said: 452 4.1.0 <myaccount@icloud.com> server temporarily unavailable
   AUP#MXRT (in reply to MAIL FROM command)”.

Occasionally they say “host tmo-east.mx.a.cloudfilter.net[54.147.231.4]
   refused to talk to me: 421 tmo-ibgw-5001a.ext.cloudfilter.net cmsmtp too
   many sessions from 17.57.156.19 AUP#CNCT" (the '17...' IP of which represents iCloud).

Another rare one is: “host tmo-east.mx.a.cloudfilter.net[34.232.155.8]
   refused to talk to me: 421 tmo-ibgw-5003a.ext.cloudfilter.net cmsmtp
   17.57.156.8 blocked AUP#CNCT”

I know that "AUP" means "Acceptable Use Policy", which suggests the failures are related to SPAM-checks. However, these are single messages, sent across a week (one week per month), to a single phone number - they're not mass emails, forwarded, or sent to multiple phones.

I did find something that suggested that T-Mobile was going to give up on this service as it used antiquated technology, and/or that they had started charging for it if were business- or marketing-related. My messages are for work on-call purposes; however, work sends the email, and I'm choosing to send the text so my phone alerts me of the message.