PGPsendmail is a drop-in wrapper for the standard sendmail programme
which resides as /usr/sbin/sendmail on most Unix systems. It allows the
automatic encryption of outgoing messages by using the recipient's PGP
public keys. It does *not* provide for automatic decryption of incoming
messages: such a feature would necessarily involve compromising your
passphrase. However, it can provide for automatic signing of outgoing
messages by using PGPdaemon.