Version:           unknown (using 3.5.9, Gentoo)
Compiler:          Target: i686-pc-linux-gnu
OS:                Linux (i686) release 2.6.24-gentoo-r4

RFC4021 (Registration of Mail and MIME Header Fields)[1] specifies that an email may have an "Expires" header which specifies a time when an email loses its validity.
This would be a very useful feature to implement in kmail, and I think it has been implemented in some way in MS Outlook (I don't use this software but it may be useful to make the feature compatible with other clients).

This would require these changes to KMail:
1. Ability to send an email specifying the expiry date/time (e.g. 3/1/09) or using a fuzzy input (e.g. 1 day from now)
2. Highlight expired emails that have been received in the folder view
3. Option to automatically delete or move expired emails that have not been read (or perhaps ones that have been read too)
4. Ability to filter out expired emails from a folder view

The typical use scenario would be something like this:
Bob is not coming into work today and wants to inform his colleagues. He sends a one line email with the subject "Not coming in today, feeling sick" and sets the expiry to be "half a day" because after that time the message is irrelevant. 
Jane is at work that day, receives the message as normal, then deletes it as normal and keeps on working.
Jim is on holidays that day and returns to work a week later. He opens his inbox and notices a bunch of emails highlighted in red including the one from Bob a week ago. He clicks a button "Hide expired emails" and continues working with these messages hidden. He will unhide and delete them later.
Jack is a power user and has set a filter to automatically move unread expired emails to a separate folder which he periodically empties. 

[1] http://www.faqs.org/rfcs/rfc4021.html