Your website is unable to send emails (new user registrations, password resets, etc.).
Setup/enable SMTP authentication for the website in question so that it uses authenticated outgoing email. A guide to doing this for phpVMS is available here.
Our server does not allow PHP to send mail using the old, insecure method without authentication. Enabling SMTP means entering the username and password of an actual email account under your cPanel account into the website's setup. This is done as a way to prevent spam and ensure delivery and reliability of emails.
Note: If you need assistance setting this up, please contact support.