Restrictions on client hostnames/addresses
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