What is Email footer?

The default details in the footer are the your address that you entered when you registered your account. You can edit this info by clicking Your Profile, at the top of the screen and You Can Change footer address at the time sending mail.

It will be shown like this when you got the email. Footer address should not be blank. More information about footer address can be found at their website https://en.wikipedia.org/wiki/CAN-SPAM_Act_of_2003

Whenever you create a new campaign, the address you specify in your user profile will automatically populate in the email footer like this.