[root@mail conf]# chmod 740 /opt/zimbra/conf/clamd.conf.in
[root@mail conf]# vi /opt/zimbra/conf/clamd.conf.in

change line —> %%uncomment VAR:zimbraVirusBlockEncryptedArchive%%ArchiveBlockEncrypted yes
with —> %%comment VAR:zimbraVirusBlockEncryptedArchive%%ArchiveBlockEncrypted no

[root@mail conf]# chmod 440 clamd.conf.in
[root@mail conf]# su - zimbra
[zimbra@mail ~]$ zmclamdctl restart
Stopping freshclam...done.
Stopping clamd...done.
Starting freshclam...done.
Starting clamd...done.
[zimbra@mail ~]$ exit
logout
[root@mail conf]#

Update (on Zimbra 8.6.0) :

[root@mail ~]# su - zimbra
[zimbra@mail ~]$ zmprov mcf zimbraVirusBlockEncryptedArchive FALSE
[zimbra@mail ~]$ zmamavisdctl reload
Stopping amavisd... done.
Stopping amavisd-mc... done.
Starting amavisd-mc...done.
Starting amavisd...done.
[zimbra@mail ~]$ exit
logout
[root@mail conf]#

Source:
http://www.zimbra.com/forums/administrators/51722-content-filter-quarantined-email.html