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.
If you are connected via an anonymizing communication system, you may receive an "Invalid request" notification. Please set your connection to regular mode and try again.