After confirming that you own your e-mail address, compose a message on the site. Or you can send to a special lettermelater.com e-mail address. Either way, specify when you want the e-mail to be sent. It will go out into the world just as if you sent it yourself!
LetterMeLater's free service limits the size of attachments and how many messages you can send a month. For $19.95 you can get more messages and send bigger ones.