I only recently learned that the PGP plugin for Apple Mail has a number of useful preferences that are set directly within the PGP application (instead of set within Mail). For example, you can configure PGP to automatically sign all outgoing messages. You can also configure PGP to auto-decrypt incoming encrypted messages. (You will still be prompted for your passphrase — so it’s not completely automatic (and shouldn’t be.) When receiving an encrypted message, PGP will attempt to retrieve the sender’s public key from one of the public key servers. Nice.