Yahoo Mail is massive, with an estimated 225 million active users globally. Somehow, email marketers seem to discount this mailbox provider, thinking of them as Gmail's smaller sibling, assuming that they are somebody they barely send mail to. But…Yahoo is big! Take your typical US consumer-focused email marketing list. Roll it up by who hosts what mail, and build out those mailbox provider counts. And you'll see, from your own data, that Yahoo is big. Really big. It'll probably show up as number two or number three when it comes to mailbox provider ranking. They are, ultimately, the guardians of...