Emails can loop. As both sender and the recipient have their out-of-office turned on, but both are sending emails anyway. One party sends a message which triggers the others out of office which bounces back and triggers theirs. The error message is not sent until the out-of-office is turned off otherwise another loop would occur.

Some errors are replies to Spam emails that have been sent the out-of-office message. The spam had an incorrect or fake reply-to address and when the out-of-office message is returned to the fake address that email server sends out an 'unknown' or 'failed' back to you.

If you one of our customers and you find you have emails that are looping or you are getting strange returned emails.Use the how to forward email headers article and forward them to support at runpcrun dot com.