Restrictions on sends in HELO commands
smtpd_helo_restrictions
This parameter specifies optional restrictions on what SMTP clients can send in
SMTP HELO and EHLO commands.
The default is to permit everything.  The following restrictions
are available:
-   permit_mynetworks: permit if the client address matches
        $mynetworks.
-   reject_unknown_client: reject the request if the client hostname is unknown.
-   reject_maps_rbl: reject if the client is listed under
        $maps_rbl_domains.
-   reject_invalid_hostname: reject HELO hostname with bad syntax.
-   reject_unknown_hostname: reject HELO hostname without DNS A or MX record.
-   reject_non_fqdn_hostname: reject HELO hostname that is not in FQDN form
-   check_helo_access maptype:mapname: look up HELO hostname or parent domains.
	Reject if result is REJECT or "[45]xx text".
	Permit otherwise.
-   check_client_access maptype:mapname: see
        smtpd_client_restrictions.
-   reject: reject the request. Place this at the end of a restriction.
-   permit: permit the request. Place this at the end of a restriction.
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.