Apparently this occurs when a server is expecting to receive mail before sending. This is usually results from server authentication priorities (for example, you attempt to send mail before your “spam filter” has finished its receiving process.)
The most common reason for this error is an incorrectly configured mail client. In order to solve this issue you should reconfigure your email client to successfully authenticate to the SMTP server.
In Outlook, for example, this can be done by checking the “My server requires authentication” box in the Outgoing Server section of your mail options.