Now, since zyp.org is my domain, no one else *should* be sending that as a HELO to my mail server. This gave me the idea that I should find a way to blacklist certain HELO strings. I started searching for a qmail patch that would do that, as I didn't really feel like writing it myself if I could avoid it, and came across qregex. qregex offers a number of extensions where you can add regex filtering to kill spam. Luckily, one of those places is HELO, in the form of the badhelo file.
And, as luck would further have it, the Debian qmail-src package already includes qregex (not quite the newest version, but new enough to include the badhelo addition).
So, I went ahead and created a badhelo file, added my domain name, and tested it (it worked). Glancing through my log files, I've already found hundreds of mail attempts that have been rejected by the badhelo. I'm impressed. ;-)
Anyone using qmail, or any other MTA, I highly recommend blacklisting your own destination domain from the MTA's accepted HELO strings. I would estimate I've reduced my spam by as much as 20%-40% with this.