CharlesWorks has several types of mail servers currently online.
The easiest way to identify which you have is to simply click on which of the screenshots below looks like yours.
If you are not certain at this point (possibly you have never used webmail yet) just go into your favorite web browser (like Internet Explorer or Firefox or Chrome or Safari and so on). In the address bar (NOT the search box), type http://mail.cwbackup.com (using your own domain name, of course!) and press enter.
Then just click on the picture of the email client you use: