The "Home mailbox" option specifies the optional pathname of a mailbox file relative to a user's home directory. The default mailbox file is /var/spool/mail/user or /var/mail/user. Specify "Maildir/" for qmail-style delivery (the / is required). This option sets the "home_mailbox" postfix variable.
The "Mail spool directory" option specifies the directory where UNIX-style mailboxes are kept. The default setting depends on the system type. This option sets the "mail_spool_directory" postfix variable.
The "Mailbox command" option specifies the optional external command to use instead of mailbox delivery. The command is run as the recipient with proper HOME, SHELL and LOGNAME environment settings. Exception: delivery for root is done as "$default_user".
Other environment variables of interest: USER (recipient username), EXTENSION (address extension), DOMAIN (domain part of address), and LOCAL (the address localpart).
Unlike other Postfix configuration parameters, the "Mailbox command" option is not subjected to "$parameter" substitutions. This is to make it easier to specify shell syntax (see example below).
Avoid shell meta characters because they will force Postfix to run an expensive shell process. Procmail alone is expensive enough.
This option sets the "mailbox_command" postfix variable.
IF YOU USE THIS TO DELIVER MAIL SYSTEM-WIDE, YOU MUST SET UP AN ALIAS THAT FORWARDS MAIL FOR ROOT TO A REAL USER.
mailbox_command = /some/where/procmail mailbox_command = /some/where/procmail -a "$EXTENSION"
The "Mailbox transport" option specifies the optional transport in master.cf to use after processing aliases and .forward files. This option has precedence over the 'Mailbox command', 'Fallback transport' and 'Luser relay' option. This option sets the "mailbox_transport" postfix variable.
The "Fallback transport" specifies the optional transport in master.cf to use for recipients that are not found in the UNIX passwd database. This parameter has precedence over the "Luser relay" option. This option sets the "fallback_transport" postfix variable.
The "Luser relay" option specifies an optional destination address for unknown recipients. By default, mail for unknown local recipients is bounced. This option sets the "luser_relay" postfix variable.
The following expansions are done on luser_relay: $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.