I use email to txt alerts and messages are timing out when sent from my alert sever to some addresses.

I am periodically receiving successful sends.  See below:

Message sent to mx3b.txt.att.net at 166.216.152.132 using TLS1.2 with AES256

The unsuccessful ones are going to a different server.  See below:

Reason: [{LED=450 4.2.0 <######################> - XXXXXXXXXXXXXXXXXXXXX- Service temporarily unavailable};{MSG=};{FQDN=mx3a.txt.att.net};{IP=166.216.149.129};{LRT=12/27/2022 8:41:30 PM}]. OutboundProxyTargetIP: 166.216.149.129. OutboundProxyTargetHostName: mx3a.txt.att.net

It appears this server is not responding correctly.

I guess I am just talking to myself.  The issue is still the same.  The server mx3a.txt.att.net is still not responding to O365 Exchange servers.  It will periodically start working when the DNS resolution times out and it picks a different MX server in the pool, but any time messages are sent to mx3a.txt.att.net.  The same message appears.

It would be great if someone at ATT would check and see their server is down or not responding and fix this issue for lots of people.

same problem 

Reason: [{LED=452 4.1.0 ... sender rejected (too busy for now)};{MSG=};{FQDN=mx3a.txt.att.net};{IP=166.216.149.129};{LRT=3/5/2023 10:15:20 PM}]. OutboundProxyTargetIP: 166.216.149.129. OutboundProxyTargetHostName: mx3a.txt.att.net

4.1.0 ... sender rejected (too busy for now

Is this a hard error?  If you try again 15 or 20 minutes later do you get back the same error?  Are you also using Outlook 365 as your mail service or someone else?

i only sent to 1 number but got it

Reason: [{LED=450 4.2.0 <email> - ZmsVpsXFJY8HhZmsWpln45 - Service temporarily unavailable};{MSG=};{FQDN=mx3a.txt.att.net};{IP=166.216.149.129};{LRT=3/8/2023 6:02:19 AM}]. OutboundProxyTargetIP: 166.216.149.129. OutboundProxyTargetHostName: mx3a.txt.att.net

Are you also using Outlook 365 as your mail service or someone else?

yes, I use office365

As you can see above, Microsoft/Outlook 365 mail servers seem to have issues consistently communicating with AT&T mail servers.  In at least one case, someone ran down the issue to an issue with how the sending e-mail domain got modified during some proxy operations.  I would suggest you work with Microsoft 365 support as the sender asking why AT&T is rejecting e-mail from their mail servers.

@JefferMC

That sounds a little backwards.  Instead of asking Microsoft why ATT is doing something.  I am asking ATT why it is taking the action.  I can then go to Microsoft and give them the answer.  Otherwise MS is just guessing at why ATT is doing it. 

As I shared in my first post it shows that the mail server {FQDN=mx3a.txt.att.net};{IP=166.216.149.129};{LRT=12/27/2022 8:41:30 PM}]. OutboundProxyTargetIP: 166.216.149.129. OutboundProxyTargetHostName: mx3a.txt.att.net is not responding.  

When messages are sent to other servers, the messages flow fine.  This means that ATT server (mx3a.txt.att.net) is not responding correctly.  

There is nothing Microsoft can do about that.

I would like ATT to either get their server functioning properly or adjust the DNS settings to not point to that server as an alternative to send mail.