SMTPSpy Proxy is a "dumb" SMTP proxy that waits for inbound SMTP requests and forwards all them directly to an external SMTP server, while outputting all e-mails passing through to a specified disk location, for monitoring purposes.
(Sorry folks, this is an old out-dated project, please do not use.) This provides a way to process emails and invoke your own custom actions. Written in Java. Comes with bounce handlers to process bounced mail, reads from disk or POP (or custom).