[exim] How to stop spoofed "From" address

I get a lot of junk mail to my email address, me@???
<mailto:me@mydomain.com>, with the header "From: me@???
<mailto:me@mydomain.com>". Obviously the "From:" is spoofed, because I'm
not sending the spam. The only way my server should except an email with
the header "From:*@mydomain" should be if my authenticated MUA or a
local script (my newsletter php program) sends it.

There is no good reason I can think of that, another server should be 
sending me email "from" myself.    Is there a way to force Exim to deny 
these types of incoming junk emails at SMTP time?  -- I assume this 
would be done with some form of ACL?

thanks...