DKIMproxy is an SMTP-proxy that signs and/or verifies emails, using the
Mail::DKIM module. It is designed for Postfix, but should work with any
mail server. It comprises two separate proxies, an "outbound" proxy for
signing outgoing email, and an "inbound" proxy for verifying signatures
of incoming email. With Postfix, the proxies can operate as either
Before-Queue or After-Queue content filters.