The problem is solved:
1. You need to go to: /opt/zimbra/ssl/zimbra/ca directory and you need to copy the ca.pem file to a safe location.
2. You need to use this codes and run on the file you copied:
$ openssl x509 -in ca.pem -inform PEM -out MyCertificatecrtcer -outform DER
3. Now, you may need to copy the new MyCertificatecrt.cer file to your Windows Phone.
4. With the use of your phone’s File Manager, you can now click on the MyCertificate.crt file and you should receive a message that it was successfully imported to your device.
5. Lastly, you need to enable SSL in ActiveSync on your phone.
Taken from:
Error with self-signed certificate: Unable to sync Windows Phone | Techyv.com