This error message means your email message could not be sent.
To fix this you need to make a simple change to enable SMTP server authentication in the email client you're using.
SMTP stands for Simple Mail Transfer Protocol, which is a process that enables the sending of email. Authentication is a method of identifying the origin of emails that stops them being sent anonymously and helps to combat spam.
To protect our email server from abuse by spammers, we only allow our own customers to use it. We do this by insisting that all emails sent through BT Mail have SMTP authentication to prove where they came from.
Most spamming activity originates from users of unauthenticated email. By removing the ability to send emails without authentication, spam should be significantly reduced.
Setting up an email program and enabling SMTP authentication is easy. SMTP authentication is usually switched on by ticking a box called "My server requires authentication" in the settings of your email program.