I knew that already. That wasn't my question. I already installed and configured my mail client, roundcube. It's working. What I was trying to do was put a plugin that allows you to change your password from within an account.
By the way, I think I've answered my own question by doing this manipilation. I know what the settings to enter are. But now I've got a new question. I was trying to set up the plugin, making it work. I was modifying the main configuration file suddenly getting an error message ssaying my account was suspended for high resource usage. I didn't understand. I wasn't doing anything resource-intensive. I was modifying a text file! So anyway, I went to my account panel and got it un-suspended automatically. Then I started work on my plugin again. After two minutes, the same message! I don't understand. Why? The only possibility I can consider is that when testing if the plugin was working, the load time was immense. I waited for it to work, until I got my suspension message.
What am I doing that's wrong?
And what happens after three warnings?