You can manage the Webmail Classic settings for multiple mailboxes at once through the Control Panel > Mail Manager > Edit global mail settings -> Webmail settings.
You need to mark the checkbox in front of the setting you want to modify. This will allow you to change the corresponding value. You can change multiple settings with a single update. Once you have specified the new values of your choice, you need to click the Update button. You can check the Help section of the Webmail Classic interface for more details on the different options you can modify.