Sailthru provides all your website email delivery needs, ensuring Inbox delivery for transactional and mass mail. Sailthru started out as a MySQL-powered transactional-mail service. Starting in 2009, we migrated to the document-oriented nosql database MongoDB. Moving entirely to MongoDB has allowed us to build complex user profiles to power behavioral-targeted mass emails and onsite recommendations. How and why we made the move, and how we use MongoDB today.