System Administration Guide: Network Services

.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, 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 alias is checked against the alias and the .forward+detail files. If no matches are made, the mail falls back to delivery to, but the user is able to see a change in the To: mail header.