Here's a weird one. We have a domain in Office 365. There are a few issues but in particular we have 1 user who gets repeated NDR's on sent emails, that get received by the intended recipient, regarding an unrelated recipient who is unknown to the user, and who is not anywhere in the email or the headers. The error is this:
550 5.0.350 Remote server returned an error -> 554 delivery error: dd This account has been temporarily suspended. Please try again later. - mta4145.mail.gq1.yahoo.com
Whether the user exists or not is beside the point, no one is trying to send to them. I'm not sure what information I need to provide to enable troubleshooting and still be able to mask identifying information.