simscan is a program developed by Inter7, the same people who wrote vpopmail, qmailadmin, and several other programs which are commonly used on qmail servers. It serves the same purpose for which many people use qmail-scanner, however because it’s written in C instead of Perl it runs much more quickly, making it a better choice for high-volume mail servers.
I’ve written a post which links to troubleshooting page of John M. Simpson for the problems people commonly run into when installing simscan.