My server was fine for the past few months. However, starting 2 days ago, some client complaining they can't received any emails and the sender's emails was bounced. Here's the example of bounced mail:
I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.
Final-Recipient: rfc822; john@mydomain.com
Action: failed
Status: 5.7.1 (Command rejected)
Remote-MTA: dns; plesk.myserver.com
Diagnostic-Code: smtp; 550...
Postfix 550 5.7.1 Command rejected
I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.
Final-Recipient: rfc822; john@mydomain.com
Action: failed
Status: 5.7.1 (Command rejected)
Remote-MTA: dns; plesk.myserver.com
Diagnostic-Code: smtp; 550...
Postfix 550 5.7.1 Command rejected