For R4 relay control: set SMTPMTA_REJECT_RELAYS=1 set SMTPMTA_HELO_DOMAIN_VERIFY=1 set SMTP_MTA_DENIED_DOMAINS=<textfile>
Where that <text file> contains a newline seperated list of domains that you do not wish to accept mail connections from.
This Setting is Introduced in version 4.6.3