I'm posting this as a suggestion because I don't think it's currently possible.
I would like the ability to pass mail that's already in somebody's Inbox or other folder through:
(a) the user's user-defined filters, and/or
(b) administrator-based spam/antivirus filters
As both I (as administrator) and end-users become more proficient in setting filters, and the Baynesian filters become more accurate, it would be very useful to have a second opportunity to re-examine emails that made it past filters as they existed at the time.
For example, I have a user account with over 1,000 emails in the Inbox, because the user enabled the "leave mail on server" option in her email client. I would like to pass those emails through the same body of tests as though they were just being retrieved now, to screen out spam etc. from her inbox.
Or, an end-user may have gone up and set up new folders to organize their incoming Webmail, and now want to apply newly-defined content filters against their large Inbox to move the old, existing Inbox emails into the appropriate new folders.