`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.