To send a letter and provide the user with an alternative to complaining about spam, a link to unsubscribe from the mailing list should be present. You will not be able to complete editing or send a letter without it.
Add the following line of code to your email for an unsubscribe button to appear:
Insert the line before
Save the changes. Now, after a user clicks on this link, they will be unsubscribed from your mailing list.