re :receive an "undelivered mail, return to sender" reply together with a lot of jargon Depends upon what else that message said. It usually says more than "undelivered mail, return to sender". The info gives an explanation of the issue, but I'm only guessing at possibilities as I don't know what else was mentioned in the email. Possible causes: If the email address was incorrect then it will get bounced back because there is no destination; assuming the incorrect email address was not actually used by someone else. If the recipient has used up their server quota then it cannot be delivered. Your email has been blocked by the recipient server as being classified as spam.