Overview

When reviewing the Message Delivery Log you see the Status Reply:

550 5.1.1 user unknown

Environment

• GFI MailEssentials
• All supported environments

Root Cause

Recipient Filtering was enabled on the mail server and blocking email.

Resolution

In order to avoid these situations, we have the following options:

1. GFI MailEssentials should have the Directory Harvesting filter enabled on Simple Mail Transfer Protocol (SMTP) level or not try to forward emails of non-existing users to the mail server.
2. Recipient filtering should be disabled on the recipient mail server. For more information on Recipient Filtering and how to disable it, please see the following:

If the mail server is Microsoft Exchange 2007:

• Recipient Filtering
• Configuring Recipient Filtering

If the mail server is Exchange 2010:

• Understanding Recipient Filtering
• Enable or Disable Recipient Filtering

If the mail server is Exchange 2013 or 2016:

• Recipient filtering on Edge Transport servers
• Manage recipient filtering on Edge Transport servers