Postmark validation is a computational proof that a client applies to outgoing e-mail messages. Postmarks help recipients distinguish legitimate e-mail from spam. When a recipient has postmark validation enabled, its spam filter parses each incoming e-mail message for a postmark header.
An e-mail message with a postmark header is less likely to be spam than one without a postmark header. This is because a computer does not require significant processing time to solve an individual computational postmark, but the processing time required to do so for large numbers of messages is expected to be prohibitive. This computational cost is expected to discourage spam senders. For examples of postmarked e-mails, see sections 3.1 and 3.2.