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.
What causes the error ‘SMTP AUTH is required for message submission on port 587’ in cPanel/WHM servers
With the latest cPanel/WHM update, email accounts that were working fine till now, may find their mails getting undelivered, showing this error message in the Exim log:
2018-01-17 05:57:40 H=(cruxserver12.com) [174.136.14.87]:63927 F=<zimphost@gmail.com> rejected RCPT <technical424@gmail.com>: SMTP AUTH is required for message submission on port 587
2018-01-17 05:58:09 H=(cruxserver12.com) [174.136.14.87]:63932 F=<zimphost@gmail.com> rejected RCPT <technical424@gmail.com>: SMTP AUTH is required for message submission on port 587
This has happened due to a latest security feature implemented in cPanel/WHM – ‘SMTP AUTH’ requires SSL by default.
To combat spamming in servers and to provide secure email transmission, email clients that try to send mails without using secure transmission will be denied mail delivery through the server.
The option that determines this is ‘Require clients to connect with SSL or issue the STARTTLS command before they are allowed to authenticate with the server’, which is set to ‘On’ by default, in WHM -> Exim Configuration Editor.
Setting it to Off will fix this issue :)