First off, I'd really recommend removing yourself from the whitelist. It really will help.
Secondly, (note this won't work on items sent from webmail) if you have a static IP you could stick a high priority (ie low number like -50) filter on:
Received: from [<external-IP>] (helo=[<internal-IP>])
or just
Received: from [<external-IP>]
Heh, I've just tried it out and it works
As it happens I've got it on a low priority, only just above a not USER_IN_WHITELIST check.