.forward+detail File

Another extension to the forwarding mechanism is the .forward+detail file. The detail string can be any sequence of characters except operator characters. The operator characters are .:%&!^[]+. By using this type of file, you can determine if someone else is using your email address without your knowledge. For instance, if a user tells someone to use the email address sandy+test1@example.com, the user would be able to identify any future mail that was delivered to this alias. By default, any mail that is sent to the sandy+test1@example.com alias is checked against the alias and the .forward+detail files. If no matches are made, the mail falls back to delivery to sandy@example.com, but the user is able to see a change in the To: mail header.