Prerequisites

Best way is to first install Zentyal.

Then do the follwoing steps in this (!) order:

FIRST test only (1) ONE domain, (2) only with a test user and (3) only with simplest settings in your email client (no ssl, tls, plain passwords, ...)
Make shure your provider has an MX record for you (test with http://mxtoolbox.com).
Make shure you have an virtual mail domain alias configured in the DNS section of zentyal that matches the MX record above
Install, activate and configure the mail system
Enable imap and pop3 access
Test the zentyal mail system using a test user and a virtual mail domain (the user name for mail is myUser@myDomain.com)
Install and activate the groupware module (this is in fact zarafa)
Disable the imap and pop3 access on the zentyal mail module (don't deactivate the module itself)
Enable the imap and pop3 gateways in the groupware module (ervery service which is activated here MUST BE DISABLED IN THE MAIL MODULE!!!)
Alter your main.cf on postfix so that you can send emails via your E-Mail-Client (see #Postfix section). If you don't do this, you only will be able to send E-Mails through the zarafa WebAccess (http://yourip/webaccess)!
Configure your E-Mail-Client with user name of the zentyal user (WITHOUT @myDomain.com!)

You should be able to send and receive mails through the webinter

Postfix

Add your server ip to the section of my_networks in your postfix settings

vim /etc/postfix/main.cf

# my_networks = 127.0.0.1/8
my_networks = 127.0.0.1/8 73.34.666.33/21

Or you won't be able to send emails through you email client from external places (Outlook, Evolution, Thunderbird, ...)

You'll easyly get the IP and subnetmask of your system logging into your system an type

ifconfig

Not recommended and forbidden for most providers.