Outbox
The Outbox web part displays a list of messages that the current user sent to other users of the website via the internal Messaging module. Listed messages can be filtered based on their subject and recipient and various actions can be performed with them. New messages can also be created and sent using the web part.
The web part belongs to the Messaging module. Full documentation of the module can be found in Kentico CMS Developer's Guide -> Modules -> Messaging.