How to Have Fan Mail Pop Up on Tumblrby Laurel Storm
Fan mail is Tumblr's equivalent of a private message system. It is always available, even if the owner of the blog to which you're sending the message has deactivated the "ask a question" feature for it. Unlike asking a question, you can't send fan mail anonymously -- a message sent through this system is always tagged with the username of one of your blogs, although you do have the option to choose which one you use. Furthermore, the system has restrictions in place that prevent you from sending a message through it unless you meet two specific criteria.
Follow the blog to which you want to send the message. Although a Send Fan Mail link appears on the information pop-up even for blogs you don't follow, you will receive an error message if you actually try to send fan mail to those blogs.
Wait for two days. Because the system is designed to prevent spam, simply following a blog isn't enough. If you haven't been following the blog to which you want to send fan mail for a minimum of 48 hours, you'll receive an error message.
Open the main page of the blog to which you want to send fan mail while logged in to your Tumblr account. If you're following that blog and you've waited the requisite 48 hours, you'll see an envelope icon next to the Unfollow and Dashboard buttons; click that icon to open the fan mail screen.