When someone try to send a mail from gmail to your webmail, if they see this error, there are several causes for this error.

550 Please turn on SMTP Authentication in your mail client. mail-oi1-f180.google.com [209.85.167.180]:44894 is not permitted to relay through this server without authentication.

  • SPF record can be missed or wrong. Add spf record in your DNS and controll your mail on this address: https://toolbox.googleapps.com/apps/checkmx/
  • Your cPanel mx settings can be wrong. MX records must be Local. To fix this settings: cPanel>EMail>Email Routing>Select Domain>Configure as “Local Mail Exchanger”
  • Your SSL certificate may be expired.