[Mimedefang] Block emails pretending to come from my domain

[Mark Penkower]

My company is getting a lot of messages (some new virus with infected zip 
files) that
have forged "return path" headers.  The emails pretend to come from my
organization.

I want to be able to block these messages.  The problem is that I have some
legitimate users who send emails to our domain from outside of the 
organization.
They use their isp's mail server to send the mail out, however, they have 
our domain
(user at roycenet.com) as the "reply to" setting.

I therefore have 2 questions:

1 - Is there a way to block all emails coming from outside of roycenet.com, 
including
      people forging the email addresses of legitimate users who send emails
      to our domain from outside of the organization, but at the same time 
allowing
      the legitimate users to send emails to the organization?

2 - If that is not possible, can I block all people forging the email 
addresses of
       legitimate users who send emails from outside of the organization, 
but keep
       a safe list of people who don't get blocked?


Thanks


Mark Penkower