I installed Pro version, set up the users and domains, but each time I attempt to retrieve mail, or access the server using Web interface, my password gets rejected. What am I doing wrong?

A Pro version of server has true support of local domains: you can have two users with same user name, but separate domains. Server has to tell the difference between two users with same user name, but different domains, and domain name becomes a part of user name. It means, you should use username@domain.com format for user name, rather than just username, unless user belongs to blank domain, or to the domain which is aliased to blank domain. You also can use username#domain.com or username$domain.com, instead of username@domain.com since some email clients (including Netscape), do not like @ symbol in user name. If you do not like to use user names with domain names, because you think that they confuse your users, then move the domain name as an alias of blank domain.