title: How to send a request to DB after list-unsubscribe action (mailto)
body: <p>I can't understand the algorithm after the user clicks to unsubscribe button.
  A general algorithm as I understand:</p>
  <ol>
  <li>If an email does not exist in the unsubsriber's list my backend send an email to the user</li>
  <li>User reads the received email and clicks the unsubscribe button.</li>
  <li>Email client send email via mailto:unsubscribe@domain</li>
  <li>Some algorithm to process emails to unsubscribe@domain...</li>
  <li>Loop to step 1.</li>
  </ol>
  <p>How to process emails from unsubscribe@domain? I am understood how to process user unsubscribe requests if List-Unsubscribe is a URL link but this feature doesn't be supported by many email clients.
  In my email server, email filtering uses sieve language but this one seems doesn't support launching bash scripts.</p>