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Additional validations that can be run to reject spam are checking the originating domain for MX entries and the originating IP for a reverse DNS entry.
A mail exchanger record (MX record) is a type of resource record in DNS that specifies a mail server responsible for accepting email messages on behalf of a recipient's domain. Most of the times, the same mail server is responsible for sending email messages too, and therefore enabling MX checks will protect you from unsolicited messages from spammers.