This error is caused when you have not configured your email software or device with our proper settings with regard to SMTP Authentication.
SMTP Authentication means that our server requires your software to provide a username and password each time it attempts to send mail. This helps combat spamming.
In your email settings, there is usually a checkbox for this called something like "Server Requires Authentication" - make certain this is checked. In some cases, you might need to also provide your username (email address) and email password to turn on authentication in your email software.
Here are some nice visuals of how to do this in popular email software -
Here is a video example of this setting for iPod/iPad -
Unable to send email; email software reports "The message could not be sent because one of the recipients was rejected by the server"
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