Several of the domains I own have been harvested by spammers, and as a result I am receiving high volumes both of direct spam and of bounces.

Would it be possible to have a To-address filter which worked as follows:
- Anything sent to whitelisted addresses (e.g. user1[at]mydomain.com, user2[at]mydomain.com) is treated as normal mail, checked and categorised as at present.
- Anything sent to any other addresses at that domain (e.g. sales[at]mydomain.com) is blacklisted and categorised as spam

Any other ideas on combatting this problem would be welcome.

Although you would seem to be using a SpamCop.net account (please see Spammers love Forum name = e-mail address ) .... I'm not sure that your question is actually dealing with that SpamCop.net account ..... What you seem to be asking seems more to deal with the e-mail configuration at your Domain host ..... basically you are stating that you have a catch-all system setup, i.e. you will receive anything ... yet what you are suggesting is that you would actually define only certain accounts to receive e-mail, a non-existing address would be rejected, which is more of the norm these days.

You need to set :fail: in place of catchall for your account. Emails that come to your address will go through but those which are sent on non existent addresses will get bounced.

Other method of finding senders on non existent addresses and blocking them would be very complicated and it would utilise more CPU to implement such thing.