`luser_relay`

This parameter specifies an optional destination address for unknown recipients. By default, mail for unknown local recipients is bounced.

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.

Examples:

- `$user@other.host`
- `$local@other.host`
- `admin+$local`