‘550 5.1.1 User unknown’ is one of the common issues reported by users sending mails to Exchange servers. 550 error means there is some problem with the recipient server which makes it unable to deliver the mail.
In our role as Website Support specialists for web hosting companies, we’ve seen senders sending mails from other servers such as Exim, Postfix, etc. to Exchange servers, seeing this error in the bounce message:
550 5.1.1 User unknown (in reply to RCPT TO command)