Evolution will automatically check any incoming GPG-signed messages for validity. If Evolution cannot GPG verify a message due to a missing public key (or tampering), it will end with a red banner. If the message is verified but you have not signed the key either locally or globally, the banner will be yellow. If the message is verified and you have signed the key, the banner will be green. When you click the seal icon, Evolution displays a dialog with more security information about the signature. To add a public key to your keyring, use the search function along with the key owner's email address: gpg --keyserver pgp.mit.edu --search email address. To import the correct key, you may need to match the key ID with the information provided by Evolution.