I discovered that if I enter a recipient but do not click Add, then when I click Submit it adds the recipient and re-displays the message but it does not send the message.
If you Preview your message, the recipient is automatically added.
If you don't see:
Your message has not been sent.
Once your message has been sent, you will see it in your Outbox.
Once your message has been read by at least one of the recipients, you will see it in your Sent messages.
