Unibox provides a backend service, so-called Domain Avatars, to check for icons of the domains of the sender address of your emails. Many company logos such as Amazon and Twitter icons will be displayed as avatar.
You can provide a Domain Avatar for your own website. Unibox tries to fetch a favicon or apple-touch-icon in the document root. If no image could be found Unibox parses the index page of your website in order find a location of such a file in the header section. The image should be at least 64 px in height and width but no more than 152 px. There is a cache of 4 weeks.