When I send an email from my gmail to an email at my domain address, I observe this error message responded back:
550 5.1.1 : Recipient address rejected: User unknown in virtual alias table
Why do I see this error message and how to fix it?
This error message is coming from your mail server (in your case, postfix). This means that the target address does not exist. To find out what email addresses exist on your system, here is your command:
cat /etc/postfix/virtual