I was trying to help a client upgrade their installation from 5.x to 6.x but ran into problems getting DSPAM working. After some digging, I found that Amavisd >2.6.3 changed how it handles anti-spam scanners. They've added an array of anti-spam scanners and Amavis::SpamControl::ExtProg module.

I've added the following to the amavisd.conf.in below the
%%uncomment LOCAL:amavis_dspam_enabled%%$dspam = '/opt/zimbra/dspam/bin/dspam'; line.

--- New configuration information ---
@spam_scanners = (
['SpamAssassin', 'Amavis::SpamControl::SpamAssassin' ],

%%uncomment LOCAL:amavis_dspam_enabled%% ['DSPAM', 'Amavis::SpamControl::ExtProg', $dspam,
%%uncomment LOCAL:amavis_dspam_enabled%% [ qw(--stdout --classify --deliver=innocent,spam --mode=tum
%%uncomment LOCAL:amavis_dspam_enabled%% --tokenizer=chained,noise --user), $daemon_user ],
%%uncomment LOCAL:amavis_dspam_enabled%% # use option --feature instead of --tokenizer with dspam < 3.8.0
%%uncomment LOCAL:amavis_dspam_enabled%% mail_body_size_limit => 64000, score_factor => 1,
%%uncomment LOCAL:amavis_dspam_enabled%% ],

--- End of configuration information ---

I hope this helps others that may face the same problem.