Mail Sanctuary is a small addon designed to solve a bug that exists in WoW at the moment. When you ignore a user and they send you mail - you will still receive a mail icon; but no mail will show at the mailbox leaving that icon to show in all its obnoxious glory!
Instead with Mail Sanctuary when you visit a mailbox and you have mail from an ignored user, the icon will be automatically removed allowing you to move on with your day!
The mail icon keeps reappearing!
Unfortunately in the WoW API there's no way to prevent this for good without making a complete overhaul to the system. As a result the mail icon will reappear whenever you zone or log back in. Visiting a mailbox will immediately clear this.