After enabling the mail filter the default /etc/amavis/conf.d is moved to /etc/amavis/conf.d.old and a single file is created by zentyal in conf.d : 01-zentyal
The issue is that this file is mostly empty:
# Turn on SpamAssassin debugging (output to STDERR, use with 'amavisd debug')
$sa_debug = 0;
#-------------
1; # insure a defined return
#Sections:
# Section I - Essential daemon and MTA settings
# Section II - MTA specific
# Section III - Logging
# Section IV - Notifications/DSN, BOUNCE/REJECT/DROP/PASS destiny, quarantine
# Section V - Per-recipient and per-sender handling, whitelisting, etc.
# Section VI - Resource limits
# Section VII - External programs, virus scanners, SpamAssassin
# Section VIII - Debugging
#
I tried on a fresh install on a VM and the same issue occurs.