[CentOS] user names

I came across /etc/mail/virtusertable.  I am using fresh install of centos
7.
I added my silly user as:
silly at my.com              silly

did the make and service sendmail restart.
But its still not working. Getting rejects as not valid just like before. I
presume this would use the PW for "silly" user.

Thought I was onto something there.

Jerry