Check for a typographical error in the entered email address. Otherwise, the user could be aliased to a nonexistent email address in /etc/mail/aliases or the user's .mailrc file.
You cannot mail to a user that has capital letters in its name. sendmail(1M) converts all the capital letters to lowercase before attempting to find the user. Because UNIX is case sensitive, it finds no user name on the system with all lowercase letters, so it displays the User unknown message.
As a workaround, make sure all user names are composed of only lowercase letters.