You can enable restrictions so that the MTA does not accept messages when non standard or other disapproved behavior is exhibited by an incoming SMTP client. These restrictions provide some protection against ill-behaved spam senders.
By default, the following SMTP protocol violators are restricted:
Hostname in greeting violates RFC
Client must greet with a fully qualified hostname
Sender address must be fully qualified
The following DNS based restrictions can also be enabled:
Client's IP address
Hostname in greeting
Sender's domain
Important: Before setting these restrictions, identify what type of mails is received outside of your mail system. If a mail system you want to receive mail from is poorly implemented, you may have to compromise your restrictions to accommodate their messages.