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not permitted to relay through this server without authentication

How to fix ‘SMTP AUTH is required for message submission on port 587’ error in cPanel/WHM servers

Mail servers have an SMTP authentication feature enabled in them, to validate users who try to send mails through that server.

The ‘SMTP AUTH’ error usually happens when users try to send mails without properly authenticating their mail account in email clients such as Outlook or Thunderbird. (more…)