The VPOP3 Webmail service provides Webmail, administration and CalDAV services to users & administrators.
You access the Webmail service by going to http://<server address or name>:5108
(The :5108 is because, by default, VPOP3 installs its Webmail service on port 5108 so as not to conflict with an existing Web server on the same computer. There is no reason not to change it to use port 80 (or 443 if you are using encryption in VPOP3 Enterprise)
The login details for both Webmail and administration are usually the same as your login details for accessing the POP3/SMTP/etc services.