Sometimes spammers will send an email with a source email address of the domain being protected.
Email From: firstname.lastname@example.org Email To: email@example.com
This is done in an attempt to bypass spam filtering, as people will sometimes white list their internal domains by name. White listing is best done by IP address.
Expectations, Requirements If the domain has an SPF record, then this option can be enabled to block this kind of email. DKIM can also be used.
It is not a requirement to have either of these so not every domain will use them. In this case it will not be an option to use them. Enabling them is done via the session profile.
If these are not options then this can also be blocked using access rules. - the first rule(s) will be to allow outbound email from that domain. You will need to create enough rules to allow all valid servers to send email using that domain as the source. - the last rule will be a catch all to prevent everyone else from sending emails form that domain.