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 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