I have a ticket open for this.  I am receiving the messages when sent from Gmail, but not Yahoo, or my company’s mail server.

I'm in the same boat. I can receive from Gmail, but not from any other email or from my work's server.

Any updates?

Same here, i am sending with my own email server and all was working more or less until 9/4/23.  

My problem got resolved exactly 30 days after it started  No matter how many times i called or escalated, would anything happen.  I almost jumped ship. 

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.nete35.85.199.61]
   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.neto54.147.231.4]
   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.

I have been battling this for over a year.  Sending from various email accounts no longer work while others work.  @gmail.com, @hotmail.com,. @aol.com, @yahoo.com, @edu.com, @outlook.com.  This is going to my phone number. I can send from my phone just fine.  It is maddening.  What I see in my email “Your message couldn't be delivered. Despite repeated attempts to contact the recipient's email system it didn't respond.” I also have texts from American Messaging and so far those still work (what my company uses for alert pages).  I gave up working with TMO support.  Supposedly it was escalated to some higher tier support but I never was contacted and Each time I contact them I have to start the whole troubleshooting process\discussion over. No matter if I told them this is the 100th time I have called.