For anybody posting from work, you might be interested in a product called "Disappearing Email". According to the
vendor it lets you:
determine how long messages remain readable
be confident that your messages are always under your control
specify email records you wish to make permanently unreadable
communicate with family members and friends without having your
messages become part of your company's corporate record
protect email stored on laptops by making it unreadable after a
specified period of time
It's actually a form of automatic public encryption: The "key" is stored
on a central computer, and remains valid for whatever duration the
sender has designated. During normal use, the sender composes HTML mail
(it won't work on plain text) and the software embeds a link to the
centrally-stored key and decoding software. The key allows the message
to be decoded automatically--- no password, user intervention or special
software is needed on the recipient's end.
At the end of the valid period designated by the sender, the key is
withdrawn, and the email then becomes unreadable. It doesn't actually
"disappear," but stays as encrypted gibberish that would be--- at best--
- very hard to decrypt.
"Disappearing Email" is free for personal use.