Cloud ESA - Which log? - 550 #5.7.1 Your access to submit messages to this e-mail system has been rejected.

[dfsaqwe]

One of our users is using an external application, and they are not receiving emails from it.

Error message given is 550 #5.7.1 Your access to submit messages to this e-mail system has been rejected.

I cannot find a relevant message in E-Mail Message logs so its not even getting that far.

Which log would contain the connection and rejection information?  And/or would that log contain the information why it was rejected?

[shane.dollery]

Check to make sure that the IP is listed in the HAT as an allowed sender.

[marc.luescherFRE]

Assuming the message shows up in the mail_logs I would be tempted to say that your sending IP is either blocked or throttled by the recipient mail system.

Assuming the message shows up in your smtp_logs I would be tempted to say that you are throttling an incoming sending system with rate controls or based on reputation.

[d1mach]

Sometimes happens if domain name has been recently registered due to the lack of data to estimate domain reputation.