To allow encrypted files to pass through to the recipient, The Global AS/AV setting 'Block Encrypted Files' needs to be disabled. However, with my version of Zimbra 8.06, this setting cannot be set from the Admin GUI screen. The checkbox can be unchecked but clicking on Save doesn't actually save the setting, it just reverts back to being checked. Fortunately, it is possible to set it from the command line (as the zimbra user) with the command:
zmprov mcf zimbraVirusBlockEncryptedArchive FALSE and then a restart of the server with: zmcontrol restart.
Note: You may not need to do a full restart of the server. Restarting the anti-virus application with
zmamavisdctl reload or zmclamdctl restart should be sufficient. But, for some reason, I ended up with services not restarting properly and since my email server is just for a handful of users, doing a full restart was acceptable.