it seems that spamassassin was not launched. the email was rejected by amavis.
amavisd is spamassassin, as amavisd is a daemon that uses the spamassassin libraries.
To your original question, change:
$final_spam_destiny = D_DISCARD;
$final_spam_destiny = D_PASS;
in amavisd.conf or the 50-user file in the amavisd directory (depends on the linux distribution) and restart amavisd.