If you would like your pop users to be able to change their own email passwords without having to login to the control panel, simply give them this link:
Where www.domain.com is either your domain, hostname, or IP address.
More information on this function and how you can use it via API can be found Here.
There is also a DA plugin that also gives email users the ability to change their passwords, and vacation messages, and show them their email stats, all in once place:
For automated vacation messages changes by email users, it can be implemented via API here, or just use the plugin above:http://www.directadmin.com/forum/showthread.php?t=13112
A squirrelmail plugin has been created which allows interaction with DirectAdmin showing usage, as well as the ability to change the password and vacation message: