I am no longer familiar with any environment other than Apache. I will answer with this limitation.
To answer this question, there are a lot of variables and considerations for which we simply do not have the information for.
As far as Apache is concerned, under default conditions, depending upon the Apache build and any control panel used, the first site ...
For webmail.example.com I think you can open it as example.com:2096 and in case of cpanel.example.com you can open it with example.com:2083
These are the general settings for cpanel by default Still if you have any issue let me know