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.