From version 3.4 on, Zentyal can provide an ActiveSync gateway on top of its OpenChange module.
There is one package available which provide this functionality, sogo-activesync.
You will need to have zentyal-openchange installed and enabled.
Using the command line:
sudo apt-get install sogo-activesync
Once you have installed one of the packages, you will be able to enable or disable the ActiveSync option from the OpenChange configuration on the Zentyal interface.