Not Kolab; but I did a little tutorial series semi recently about getting zpush going for a client;
https://talk.plesk.com/resources/installing-z-push.13/
In the resource section, I did a how to video on getting Zpush going, which is open source version of ActiveSync.
Instruction - Installing Z-Push
Not the greatest, but should get you in the right direction.
Just a thought - what about creating a fake domain in Plesk and leaving mail enabled? You'll be able to SMTP authenticate with an email address on the fake domain.
You can't yet as Plesk doesn't have the ability for resellers to create their own DNS template. You can vote for this feature;
DNS Template for Reseller
Think Peter nailed it with the PHP handler suggestion. Check the .htaccess file, if nothing is in there, try changing the domain to a different PHP handler to troubleshoot.
I would advise against blindly throwing credentials at a forum user. I'm sure Ales is a great guy, but you need to consider the security of your environment.