sending text to my cell phone using @tmomail.net no longer working?

Same problem using iCloud email. The failure messages go into my junk mail 100% of the time, hours after I sent the original message, so I had to keep an eye out for them.

They generally look like this:

<my-10-digit-phone#@tmomail.net>: host tmo-west.mx.a.cloudfilter.net[35.85.199.61]
   said: 452 4.1.0 <my-sending-email-account@icloud.com> server temporarily unavailable
   AUP#MXRT (in reply to MAIL FROM command)

OR, occasionally:

<my-10-digit-phone#@tmomail.net>: 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 address in the 2nd failure is an iCloud server.