If your website does anything more than just display content, chances are it sends out what are called "transactional emails" to admins and other users. This is especially true for e-commerce sites, but are part of any web app that provides even rudimentary functionality.
What are Transactional Emails?
These are emails that are part of the normal process of interacting with website. As an example, a typical e-commerce will have the following transactional emails, at a minimum: