Sendmail check FROM: header

[lyd mc]

Hi guys,

How do i check valid sender before sending mail?

example: 
    FROM: anonymous at mydomain.com

if this 'anonymous' user is not on localhost the mail should be rejected even if the user has valid login (user and password).

my sendmail uses login/plain before relaying. however they can still send mail even the FROM: header is not their name. I don't want this to happen.

Need your help guys.

Thnx.

alydio