If you have used CalDAV calendars on Mozilla Lightning/Sunbird, then the eM Client configuration is quite different, but simple.
To add a CalDAV account to eM Client
Go to Tools → Accounts
Click New Account…
Open the Calendar section and click on CalDAV
Press Next
In the
Account Address URL box, type the base
URL of your VPOP3's Webmail, followed by
calendar/<username>/. Note that the username is case sensitive and must be
exactly as it is defined in VPOP3. So, for the calendars for the
Postmaster user, enter something like
http://192.168.1.1:5108/calendar/Postmaster/.
In the User name box, type the VPOP3 account user name
In the Password box, type the VPOP3 account password (case sensitive)
Press Next
In the Account name box type a name you use to refer to this calendar
Press Next
Press Finish
Press OK
The calendars should now appear in the left column of the Calendar section in eM Client. Note that if the user has several calendars (in VPOP3 Enterprise) then all the calendars will be shown.