Action

Check for typo the entered e-mail address. Or, the user could be aliased to a non-existant e-mail 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. Since UNIX is case sensitive, it finds no username on the system with all lowercase letters, so it displays the User unknown message.

Workaround: Make sure all usernames are composed of *only* lowercase letters.