Thanks for your replies, but still does not work.
tried the following:
also tried using localhost and loopback ip addresses.
dns, mx records is fine because I had a fully functional version of the website and email server running on a windows box with roundcube, hmailserver, mysql and IIS7.5 but need to get it working on a Linux server instead.
My roundcube mail login page comes up, but unable to login.
As mentioned before, ispconfig3 installed with no errors.
Looking forward to your help.