Mail delivery failed: returning message to sender
You sent an email, and it was bounced (returned) back to you.
Messages like this are generally encountered when there is an issue on the recipient server, or the intended recipient does not exist on the remote server. In more rare situations, it could also be caused by a blacklisted server.
The returned message will have this title in the subject, and the body of the message should contain more details, which should explain why the message was bounced back to you. By reviewing these emails, it is possible to know what the issue was.