If sendmail was compiled with User Database support (USERDB) and if that database is being used (UserDatabaseSpec), you can have sendmail rewrite header recipient addresses using that database. The F=j delivery agent flag tells sendmail to look up recipient addresses in the User Database (using the mailname keyword). If an appropriate entry is found, it is used in place of the original address in the recipient headers. The process is the same as that described for the F=i delivery agent flag, except that it is used here for recipient headers.