To enable quarantine support simply change the following line in
$QUARANTINEDIR = undef; # -Q
$QUARANTINEDIR = "$MYHOME/quarantine"; # -Q
then restart amavisd.
The following will make the change for you:
sed -i.orig -e '/\$QUARANTINEDIR =/s,undef,"$MYHOME/quarantine",' \
PLEASE NOTE: This is intentionally a manual step. Before turning on
the quarantine feature of amavisd-new, be sure that there is a
sufficient amount of free space on /var/spool/amavisd/quarantine.
"Sufficient" depends entirely on your environment.
In the default configuration, files in the quarantine directory will
be automatically deleted by tmpwatch after 30 days. See
/etc/cron.daily/amavisd to adjust this behavior for your environment.