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 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.

For more information on how to set up your email client, you can refer to our tutorial:

http://www.siteground.com/tutorials/email/

SiteGround provides the best email hosting environment and we will help you with any email questions/problems you may have.