email-to-text messages to txt.att.net, Service temporarily unavailable when sending messages for 10+ days

The txt.att.net service has been failing with error 450 4.2.0: service temporarily unavailable.

​For the past 10+ days we are having issues with sending emails to text using *@txt.att.net. In this time period 95% of the messages sent to *@txt.att.net have failed with the following reasons before they time out after 24 hours.​

​Reason: [{LED=450 4.2.0 <sender email address> - cWQEp8MIx6BoncWQFpm8MN - Service temporarily unavailable};{MSG=};{FQDN=mx3a.txt.att.net};{IP=166.216.149.129};{LRT=3/15/2023 7:04:26 PM}]. OutboundProxyTargetIP: 166.216.149.129. OutboundProxyTargetHostName: mx3a.txt.att.net​

​Reason: [{LED=450 4.2.0 <sender email address> - ZHaTpinbnCRNvZHaWpHAco - Service temporarily unavailable};{MSG=};{FQDN=mx3b.txt.att.net};{IP=166.216.152.132};{LRT=3/6/2023 8:38:58 PM}]. OutboundProxyTargetIP: 166.216.152.132. OutboundProxyTargetHostName: mx3b.txt.att.net​

​How can this get resolved or who do I contact?

I called ATT support but since I do not have an ATT cell number they would discuss the issue.

My company uses the email-to-text message service to notify users.