This button is shown only if the correspondent option is enabled in the first tab of the domain settings (see chapter 7.2 General) and if the time settings for restoring deleted items are specified. The option must be enabled for each domain separately.
If the appropriate options are enabled in the domain settings, the User accounts section will show the button. Simply mark the user that has deleted an important message by mistake and all items (messages, events, tasks and contacts) received or created during the time interval specified will be moved to the Deleted items folder.
If an older item is to be restored (see chapter 18.2 Backup of user folders), you can use the archiving option; however, it is not designed for this purpose by default.