The Message Outbox page is dedicated to the sole task of letting you send a private message to another user or group. It is analogous to the "send to user" function of the Message Inbox, or to typing /msg username ... in the Chatterbox, but it provides a simplified interface just for sending private messages. The same underlying function sends the message in each of these cases, so you will see similar behavior in exceptional conditions, such as when the specified recipient does not exist, or if you attempt to send the same message twice.
In the Message Outbox you will see two fields:
Beside the To field is a submit button labeled Tell. When you press this button, the text in Say is sent to the user (or group) specified in To.
If either field is blank, no message is sent.
Learn about how to link to the Message Outbox.