You try to send a message from your local email client using your cPanel server as the outgoing mail server. You receive an error in the client and observe this error in the Exim mail log on the server:
SMTP AUTH is required for message submission on port 587
This error can appear when the local email client is not properly configured to authenticate using SMTP authentication.
Configure the email client for SMTP authentication, or send the message directly from Webmail on the server.