cryptographically identify the sender of email

 DomainKeys Identified Mail (DKIM) defines a domain-level
 authentication framework for email using public-key cryptography and
 key server technology to permit verification of the source and
 contents of messages by either Mail Transport Agents (MTAs) or Mail
 User Agents (MUAs).
 .
 libdkim is a library, developed by Alt-N Technologies, that allows
 incorporating DKIM into an existing MTA or other application. It
 provides full support for signing, verifying, and SSP. See the
 changelogs for information on compliance with specifications.