I would love to add an exception to filters. This would come in handy when using the filters to weed out spam messages and such, by allowing us to say apply this rule, except when... ie: except when it came from a registered email address.
You don't need to use filters to achieve this.
Go to Admin panel -> Emails -> Settings uncheck "Accept all emails"
This will make it so users have to be registered for accounts ahead of time.
