Integration into qmail using ifspamh
Since qmail allows users to configure their own delivery options via ~/.qmail files, an option if you have spamd running, is to use ifspamh, which is a spamc wrapper that you can use as a command in any ~/.qmail file.
If the message passed to ifspamh is suspected to be spam, it is redelivered to an address specified as the argument to ifspamh (typically an alias which is handled by another .qmail file). Otherwise, the message is usually allowed to be processed by the next line in the .qmail file. In either case, ~/.spamassassin/user_prefs is honored, and ifspamh returns the proper exit codes required of a command; see dot-qmail(8) and qmail-command(8).
A sample ~/.qmail to test mail to myuserid@myhost with ifspamh, delivering spam to myuserid-spam(@myhost), and feeding non-spam to procmail:
|/usr/local/bin/ifspamh myuserid-spam |/var/qmail/bin/preline /usr/local/bin/procmail
And a ~/.qmail-spam to handle what gets sent to myuserid-spam: