1. Go to http://yourdomain.tld/webmail. "yourdomain.tld" is the domain name of the email address you wish to connect to. The URL will transform into your server name and the secured Webmail login port like this: s###.panelboxmanager.com:2096. This is a normal behaviour.
2. Enter your full email address and password in the prompt or Webmail login page (some servers might display one or the other)
3. You will then have the choice to choose from Horde or Roundcube. Choose your favourite Webmail interface, and you'll have access to your Webmail Client.