I followed Zimbra Administrator's guide to turn on DSPAM, precisely, I did:
su - zimbra
zmlocalconfig -e amavis_dspam_enabled=true
zmamavisdctl stop && zmamavisdctl start
However, dspam does not appear in e-mail headers, nor does amavis report finding dspam. Dspam became uncommented in amavisd.conf though after these commands. I found out, that /opt/zimbra/dspam/bin/dspam was executable only by root. I changed that to zimbra, but that did not help.
I tried both on Network Edition Professional and on Open Source edition with the same result. Is something else required to start using DSPAM?