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.
The Subject line of the message will not be encrypted and should not be used for sensitive information.
To have every message signed or encrypted:
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.
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