What's the difference between mailto: and http: List-Unsubscribe?

Both mailto: and http(s): are valid List-Unsubscribe mechanisms, but they work differently.

mailto: mechanism:

• List-Unsubscribe: <mailto:unsubscribe@example.com?subject=unsub-123>
• Triggers the recipient's email client to compose and send an unsubscribe email
• The sender processes incoming emails to their unsubscribe address
• Works without requiring the recipient to visit a website
• Some clients don't support it well or prompt before sending

http(s): mechanism:

• List-Unsubscribe: <https://example.com/unsubscribe?token=abc123>
• Opens a URL when clicked
• Without List-Unsubscribe-Post, it typically opens a webpage (extra step)
• With List-Unsubscribe-Post, enables one-click POST request
• List-Unsubscribe-Post: List-Unsubscribe=One-Click

Best practice: include both:

• List-Unsubscribe: <https://...>, <mailto:...>
• List-Unsubscribe-Post: List-Unsubscribe=One-Click

This provides maximum compatibility. Clients choose their preferred method. Gmail prefers HTTPS with POST; others may use mailto.

The 2024 bulk sender requirements emphasize HTTPS with List-Unsubscribe-Post for true one-click functionality. Mailto alone doesn't satisfy the requirement.