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.
- The restrictions only apply when sending fan mail messages, not when responding to them. Once you've sent a fan mail message to somebody, she'll be able to respond even if she's not following your blog or has been following it for less than 48 hours.
- As of December 2013, fan mail can't be deactivated the way the "ask a question" feature can. When using the feature, consider that some blog owners may prefer not to be contacted by fan mail. If that's the case, there will usually be a message stating precisely that on the blog. If you see such a message, respect the blog owner's wishes instead of sending unwanted fan mail.