[TriLUG] Setting up roots email address?

[bp]

I started running Postfix at home and would like to know how you 
associate a email address with root or another system user.

When I execate a cmd like:
     cat afile | mail -s "My eml subj" recipent at domain.com

The email arrives fine but appears to be from "root" 
<root at localhost.localdomain>

How would I change this system wide to a real name and email address?

-bp