How to automatically decrypt incoming PGP emails (linux, gnupg, procmail/qmail)?

Question

linux

Ideally, you need a complete analogue of the Windows GPGrelay for Linux, so that the encryption of outgoing and decryption of incoming messages is absolutely transparent for any MUA.
It doesn't have to be exactly a POP3/SMTP relay on localhost, a wrapper script for sendmail that encrypts outgoing mail and a filter script for procmail/qmail that decrypts incoming mail (preferably adding information to the decrypted email that it was encrypted/signed like GPGrelay does).
So far, I have found only one utility for encrypting outgoing emails (kuvert), and have not found anything for decrypting incoming emails.