Check out the BigString Email service.
It allows you to restrict the allowable number of views of a message, set the message to expire after certain duration, edit, recall, change attachments, delete attachments, and much more.
I had thought how I could build an expiring/recallable mail service like this long ago when I was still in college. And the solution I could think of... one that doesn't require a new email protocol or any kind of additional support from the client's email reader software, was to render the contents of the message in an image, host that image on the email-service provider's servers, and insert that image inline in the HTML of the email message that is sent.
Later if the email sender wishes to make modifications to the contents of the sent email message, the image file on the email service provider's server can be replaced with a new one that has the new message contents rendered. To make a sent mail unavailable to the original recipients, we can simply delete the message's image from the server, or replace it with another that says that the message isn't available.
Attachments can be handled in a similar manner by storing them at Email Provider's Servers and inserting links to them in the HTML of the email message sent.
I tried out Bigstring a bit, and that is exactly what it appears to do. Bigstring, btw appears to have been written using Perl and PHP.
They have four packages available, including the free one.
Nice work dudes!