New domains access panel ( cpanel , webmail,)

[TGates]

It is much easier and possibly better practice to use an alias instead.
Ex: http://domain.com/webmail/
http://forums.sentora.org/showthread.php?tid=1214

It can be more difficult to work it as a sub domain.

This post will give you an idea of how to do it as a sub domain: http://forums.sentora.org/showthread.php?tid=495
Ex. http://webmail.domain.com/