When you send messages to multiple people in a short time span, you may be temporarily restricted from sending more messages.
Please try waiting a while before sending more messages if you receive the error, "Unable to send message".
You cannot send messages from an account whose e-mail address has not yet been confirmed. Please confirm your e-mail address before you try sending messages.
Reference: I haven't received the e-mail address confirmation email so I want it to be resent