I realized that there isn’t any interface to allow users to change their own password, Zentyal has a ‘User Corner’ web based interface to change passwords and add their own POP3 / IMAP connectors.
Also I noticed that within the POP3 connector section of the admin interface, there is no way of changing ports (eg. if I wanted to include a connector between Gmail POP3 and NS, I can not specify port 995 or any unusual / non-standard pop3 / imap ports).