After setting up POP email accounts on your domain, you can enable Webmail on your account to access your POP email accounts via the web.
To do this, login to ServerTools and select the Web Mail option on the Script Manager menu. You will be given the option to choose where webmail will be installed on your domain and other settings.
Assuming you chose the default location, you will then be able to access the webmail login page by going to an address such as http://your.domain.here/webmail where you will be greeted by a login that looks like this:
The first time you login to Web Mail with each POP email account, you will need to set the return address for that account.
To do that, login to webmail, and click on Options->Identities->Edit as shown here:
Then enter the Email and Reply-To email address that you want others to see when you send out email. Optionally, you can enter a name that will show along with the email address.