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:

http://www.domain.com:2222/CMD_CHANGE_EMAIL_PASSWORD

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.

For automated vacation messages changes by email users, it can be implemented via API here: http://www.directadmin.com/forum/showthread.php?t=13112