Email scheduler not sending in Atlas -- "Some of your scheduled messages have not been sent."

Your SMTP server requires authentication and you haven't specified the SMTP username and password that the email scheduler should use when sending the mail.

If you log in to the Atlas control panel and go to Plugin Settings / Email Schedule -> Configure, you will see the fields where you can specify the SMTP username and password. You may need to create a new email account on your server just for this purpose and use that email account's username/password.