550-Please tu on SMTP Authentication in your mail client. Why?

When I sent mail to some domains, it retued :

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

recipient@recipientdomain.com
host recipientdomain.com [222.222.222.222]
SMTP error from remote mail server after RCPT TO:<recipient@recipientdomain.com>:
550-Please tu on SMTP Authentication in your mail client.
550-mail.mydomain.com [111.111.111.111]:33832 is not permitted to relay
550 through this server without authentication.
Reporting-MTA: dns; mail.mydomain.com

Action: failed
Final-Recipient: rfc822;recipient@recipientdomain.com
Status: 5.0.0
Remote-MTA: dns; recipientdomain.com
Diagnostic-Code: smtp; 550-Please tu on SMTP Authentication in your mail client.
550-mail.mydomain.com [111.111.111.111]:33832 is not permitted to relay
550 through this server without authentication.

I set SPF correctly. Tested from several online SPF tester tools.
PRT Record is set.
MX is set correctly.
A Record for MX is set
It happened to several recipient domains. I am using the latest DirectAdmin, updated a few days ago.
How can I solve this problem?

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