I have tested with opensource zimbra 8.0.2 on centos 6-4 with widows xp – outlook express and windows 7 – outlook 2007
zimbra server hostname = desktop51.bipinpatel.com
zimbra server ip = 192.168.0.51
on zimbra server run command as root
openssl x509 -in /opt/zimbra/ssl/zimbra/ca/ca.pem -outform DER -out ca.der
now install this ca.der crtificate in windows.
On the Outlook system, double-click the ca.der file.
Click Install Certificate.
Click Next in the Welcome screen.
In the Certificate Store screen, click Place all certificates in the following store and click Browse.
Click Trusted Root Certificate Authorities and click OK.
Click Next, then Finish.
If a pop-up appears, click Yes and wait for the The Import was successful message.
now in windows, zimbra server hostname must be resloved.
if you using ip for incoming/outgoing server in outlook then it is not work there must use zimbra server hostname.
if zimbra hostname is not resolved in windows then add zimbra server hostname in windows
192.168.0.51 desktop51.bipinpatel.com desktop51