I use maildrop to do some of my filtering on my email, mostly with another domain that I used to get a lot of mailing list messages sent to. It’s also useful for filtering the results of various spam tools. In the past year, I started using a specific email address from this domain on certain sites where I wanted to use the dcortesi address, but didn’t want to put my main email on line. Ever since, I’ve been receiving nearly 50 spam emails a day. iMail on my mac does a good job of filtering those out, but I frequently access my email using other means including webmail and from my phone where iMail has no effect. So I needed a server-side solution, and decided to use my current maildrop setup. The only problem was I needed to forward mail from one domain to the other and then back again as maildrop is just set up for a specific user on my other domain. Once I did that, though, it was a piece of cake to filter out the cruft:

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if ( hasaddr(spam_catching_email@) )
        if (/^X-Spam-Flag: YES/)
                to Mail/webd_spam
        else
                to "!my_real_email@"

Cool - now my phone/webmail inbox is a lot cleaner. :)