Seems no one could help your case... actually sometimes the ppl here would not giving hand to others if they are not interested in your questions or just think that your problem is not able to be fixed~ (like my asked questions)
I am not sure, but I think the only way for the users to change their password without using the web browser is, accessing the mail server via ssh or telnet, and then changing their own password manually by using zm command! But it's not easy for the users~ So the most simple way would be through the GUI~