`mailbox_command`

This parameter 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, this parameter 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.

IF YOU USE THIS TO DELIVER MAIL SYSTEM-WIDE, YOU MUST SET UP AN ALIAS THAT FORWARDS MAIL FOR ROOT TO A REAL USER.

Examples:

- `/some/where/procmail`
- `/some/where/procmail -a "$EXTENSION"`