Unsubscribe link
The way out of your list — required by law and by every mailbox provider.
Every marketing email needs a working, obvious way to stop receiving it. Beyond the link in the footer, mailbox providers expect the List-Unsubscribe header, which is what powers the one-click “unsubscribe” button the client itself shows next to the sender name.
Hiding it is self-defeating. A reader who cannot find the link marks the message as spam instead, and a spam complaint costs you far more than a quiet unsubscribe.
Make it work in one step. Asking someone to log in, confirm and answer a survey converts an unsubscribe into a complaint.