Support encrypted and unencrypted subject/body in the same email

[rubenhensen]

Description

Add the ability to send both a secure (encrypted) and an unsecure (plaintext) subject and body in the same email. The plaintext portion helps the recipient understand who is sending and why, before decrypting.

Context

The PostGuard website already has a similar feature where you can add an unencrypted message to the email template. The Thunderbird addon should support this as well.

Goal

• Allow the user to compose an unencrypted subject/body alongside the encrypted content
• The unencrypted portion serves as context (e.g., "This is a secure message from X regarding Y")
• The encrypted portion contains the actual sensitive content