Yeah, sure.. Sorry I wrote that in kind of an hurry
So, we have couple of cpanel servers already and we were thinking of using them to manage the user email accounts.. We have a domain and we would like to use that domain as the email domain.. So whenever a user registers a email account for himself, in this case trough whmcs he would get an e-mail user@ourwebmaildomain.org. And he would be able to use the webmail trough his own cpanel account. Now he problem is that cpanel won't let us create multiple users for one domain..
And now that I have tought about this, I think it can't be possible in any way.. So we need a mail server software which would perhaps work with whmcs.. atmail is not an option, becouse it is too expensive...