`smtpd_client_restrictions`

This parameter specifies optional restrictions on SMTP client host names and addresses.

The default is to allow connections from any host. The following restrictions are available: Restrictions are applied in the order as specified; the first restriction that matches wins.

Specify a list of restrictions, separated by commas and/or whitespace. Continue long lines by starting the next line with whitespace.

Sensible examples:

- `permit_mynetworks, reject_unknown_client`
- `reject_maps_rbl, reject_unknown_client`