I really need to solve this error: #550 5.7.1 External client with IP address xxx.xxx.xxx.xxx does not have permissions to submit to this server. It is happening with random senders. Please help!!!

Do you have any SMTP devices between the Internet and your Exchange server, including firewalls that "help" you scan SMTP? Do you have a secondary MX record that goes to a non-Exchange host? What do the SMTP protocol logs say about the session?Ed Crowley MVP "There are seldom good technological solutions to behavioral problems."

where do you get this error, can you copy paste more information. if it is happening to some senders, are you able to receie email from them on yahoo or live mail?

Hi, From the error message, it seems that your Exchange server doesn't have the permission to receive the Anonymous email or the unauthenticated sender try to send the email by relaying your Exchange server. You can check the issue by using the SMTP protocol as Ed said. Thanks AllenAllen Song