GFI MailEssentials is installed on a gateway machine. When users are sending emails to the list server, all emails are rejected with an error similar to the following:
PERM_FAILURE: SMTP Error (state 9): 550 5.1.1 User unknown
The 550 5.1.1 User unkown error message is returned to the sender of an email by Microsoft Exchange server when the recipient's email address is not found in the customer' Active Directory.
This will occur if the recipient filtering is enabled on the Microsoft Exchange server and it is set up to block non-existent users. If GFI MailEssentials has Directory Harvesting enabled (which does essentially the same thing), GFI MailEssentials will catch the emails and funnel them through the List Server process before it reaches to the Microsoft Exchange server.
Use the Directory Harvesting feature of GFI MailEssentials to prevent emails being delivered to the list server where destined recipients do not exist in the customer's Microsoft Active Directory.
Directory Harvesting can be enabled as follows:
- Open the GFI MailEssentials Configuration.
- Browse to Anti-Spam > Anti-Spam Filters > Directory Harvesting > General tab.
- Check Enable Directory Harvesting Protection.