You don't email it. It's a postcard--you have to make it. How would you email a postcard? Create your 4-by-6-inch postcards out of any mailable material.
Mail your handmade postcard with your secret to: Postsecret 13345 Copper Ridge Rd. Germantown, Maryland 20874
Moved from Health (??) to "Writing and Literature".
Remember you get points for putting your question in the right category.