Speaking of public keys, here’s a SourceForge announcement for GPGrelay:
GPGrelay is a small email-relaying server that uses GnuPG (the GNU Privacy Guard) to sign/encrypt (SMTP-Relay) or verifies/decrypts (POP3-Relay) emails. This enables many email-clients to send and receive emails that are PGP-MIME conforming. This release includes bugfixes and minor feature enhancements.