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

When I sent mail to some domains, it returned :

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:

[email protected]
host recipientdomain.com [222.222.222.222]
SMTP error from remote mail server after RCPT TO:<[email protected]>:
550-Please turn 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;[email protected]
Status: 5.0.0
Remote-MTA: dns; recipientdomain.com
Diagnostic-Code: smtp; 550-Please turn 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?