You receive the following error when sending or receiving mail:
550-Please turn on SMTP Authentication in your mail client
This issue can occur for many reasons. You should consult these options for review.
1. ConfigServer Users
If you use ConfigServer Security & Firewall (CSF), there is a potential conflict between CSF and the "Restrict outgoing SMTP to root, Exim, and mailman (FKA SMTP Tweak)" Tweak Setting.
SMTP Restrictions (WHM) versus SMTP_BLOCK (CSF)
2. Mail Routing Settings
If, when sending a message from one provider to your server, you encounter this 550 error message, despite not using a mail client, the cause of this error is likely to be related to the mail routing settings.
If the MX record points to this server, you must set the mail routing configuration to local.
What is The Difference Between Remote Vs. Local Mail Exchange Within "Email Routing"?
3. Mail Client Configuration
Some users may have their mail client configured in a way where sending mail does not authenticate with a password. This can be confirmed by changing this setting:
How to enable relaying of mail from same IP of users who have already authenticated.
If this is the case, the user should adjust their mail client settings to authenticate when sending mail.
4. Mail Scripts
If you attempt to send mail from a script, and it is failing with this error, you should review the restrictions of the SMTP Restriction Tweak:
PHP Mail won't send mail or PHP Mailer certificate error
5. Transferred Accounts
If the account was recently transferred, there also may be a configuration error to review:
New or transferred account unable to receive mail: Rejected relay attempt | 550 Please turn on SMTP Authentication | $HOST is not permitted to relay through this server without authentication