The Inbox is a feature used to mail other users and receive mail from other users that can be read even if the user is not online at the time, and to see various notifications from different features on the inbox.
Note: As of version 523.0, the client "Inbox" button now redirects you to the Website inbox. You can find your matches for "Get Matched" through the Get Matched feature
The Client inbox gives off notifications for each incoming mail while you're in chat. It allows you to view all received mail, but does not allow you to view your own sent mail.
Instead of categorizing by sender, the Client inbox categorizes by most recently received mail. Unlike the website, Mobile or Next's inbox format, the Client does not allow you to see the entirety of the conversation, in which you cannot see your own sent messages.
All other inbox formats show the full conversation, including all messages you've sent to other users. This is useful to remember your conversation history, and allow you to send messages quickly, even if you cannot access the client.
The website allows you to see notifications such as Friend Requests, Missed Chat invites, and Group Invites.