Use the editor to customize the footer that is automatically added to your messages. The editor resembles that of a common word processor; the question mark button all the way to the left will help you get the meaning of every button.
Make sure you always insert an unsubscription link in your messages. This will give recipients a clear way to unsubscribe and will avoid spam complaints due to missing or non-working unsubscription link. Use the button 
to insert the link. Default footer already comes with an unsubscription link.
If you want to have a complete overview of how the built-in editor works, go to this page
Please note that this page is divided into two parts. You can freely customize the upper part ("Customizable footer") without any additional option:
On the other hand, in order to customize the "MailUp footer" you need to purchase the Private Labeling option:
More about this topic: Common issues with the unsubscription link (article taken from our Knowledge Base)