When you reblog a user's post on Tumblr, you can add your own comments in the post and reply to the original content in a public post. However, if another user wants to ask you a question privately, the message must be sent through your Ask box. Tumblr doesn't have a true native private messaging system with an inbox and an archive of sent messages, but the Ask box allows users to send you private messages to which you can send a single reply.

Step 1

Log in to your Tumblr account and click the mail icon button in your dashboard to visit your account's aggregate inbox.

Step 2

Locate the question to which you'd like to respond. If you have more than one Tumblr blog and you know which blog received the question, you can filter your inbox asks by selecting the desired blog from the drop-down menu.

Step 3

Click the "Answer" button in the post. The "Answer" button features an outline of a square with the silhouette icon of a pencil.

Step 4

Enter your reply to the question. Add tags and a subject line, if desired, and then click the "Publish" button to publish the reply to your blog as a new blog post. Alternatively, you can click "Answer Privately" to respond to the question privately by sending a response to the user's private inbox.