forward_path
This parameter specifies a search list. The first file that is found is used.
The following expansions are done: 
- $user (recipient username),
- $shell (recipient shell), 
- $home (recipient home directory),
- $recipient (full recipient address), 
- $extension (recipient address extension), 
- $domain (recipient domain), 
- $local (entire recipient localpart), 
- $recipient_delimiter.
Specify ${name?value} or ${name:value} to expand value only
when $name does (does not) exist.