Signing or encrypting messages
After you have set up your GPG key, you can sign or encrypt a message by selecting Options ▸ PGP Sign or PGP Encrypt from the message composer menu or clicking the corresponding toolbar icon.
邮件的主题不会被加密。所以，切勿在主题中填写敏感信息。
给每封邮件添加签名或加密：
Select Edit ▸ Preferences ▸ Mail Accounts.
Select the mail account to encrypt the messages in.
Click Edit.
Click the Security tab.
Select Always sign outgoing messages when using this account.
Click OK.
Click Close.