To comply with 3rd party bulk sender rules you need to ensure your messages include a one-click unsubscribe link. You can achieve this by adding the variable #UnsubscribeLink# to your mailing list messages. This will then add a customized URL for each subscriber that they can use to unsubscribe.
See the steps below for how to automatically include this link on all mailing list messages by using the message footer.
In webmail go to Domain Settings > Accounts > Mailing Lists and select your mailing list.
On the Options tab under Message Options, find "Text to show in unsubscribe link" and add your desired text (e.g. Unsubscribe, Click Here to Unsubscribe, etc).
Go to the Messages tab and click "Posted message footer"
In the modal that opens add #UnsubscribeLink# to the text box and click Save.
For better presentation you can center the link. Click the Show More button (plus symbol), click Align (3 lines symbol) and select Align Center. Then click Save.
Now when you send messages to that mailing list, the unsubscribe link will always automatically be included:
