Linux : cPanel outgoing email return “this server is not permitted to relay through this server without authentication”

By | August 21, 2021

Recently, without any change, I noticed that some cPanel servers stop sending email and return the following error:

550-Please turn on SMTP Authentication in your mail client.
550-hostname.domain.tld []:00000 is not permitted to relay
550 through this server without authentication.

What is really puzzling in that case is that this message showed up even though the sender had SMTP authentication properly set!

Here is some background information about the issue encountered :

  • Servers were running well for several years, without any recent configuration change ;
  • cPanel version where the issue has been encountered are 96.0.15 and 98.0.5 ;
  • All users mail clients where this issue occurred were properly configured for SMTP authentication ;
  • The issue is shown regardless if this is a remotely connected client, as well as from the server webmail (Roudcube or Horde) ;
  • The issue is also shown when sending a test email from the root account using the “mail” command from the shell ;
  • Restarting the Exim/Dovecot services or even the server did not helped.

It is still unclear why this condition occurred all of a sudden without any modification, and the resolution is even more puzzling, that make me believe to a software bug. Here is the steps taken to solve the issue in my case :

  1. In WHM, go to Exim Configuration Manager ;
  2. Backup the current configuration using the Backup tab ;
  3. Then reset the Exim configuration to default – at this point, outbound mail should be flowing again ;
  4. Go to the Restore tab to restore your previous configuration.

In my case, this completely fixed the issue. If in yours this still persist, review your configuration parameters (besides this particular case that I classify as a “bug”, the common issues are ;

  • Weak ciphers disabled where users have deprecated mail client that does not support modern cipher suite ;
  • Mail routing option isn’t set to local.