When reviewing the Message Delivery Log you see the Status Reply:
550 5.1.1 user unknown
- GFI MailEssentials
- All supported environments
Recipient Filtering was enabled on the mail server and blocking email.
In order to avoid these situations, we have the following options:
- 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.
- 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:
If the mail server is Exchange 2010:
If the mail server is Exchange 2013 or 2016: