According to the earlier mentioned 2010 Annual Email Census performed by Econsultancy in association with Adestra, there are a minority of companies that use email marketing as an automatic response to consumer actions on their web site: triggered emails.
These emails are usually sent as the result of a sign up or purchase on a web site.
From a marketing stand point, they typically serve to offer additional products or services to the services that were originally purchased or accessed – triggering the email.