[Gllug] quick exim question

[Minty]

I'm getting a bunch of spam of the type:

Received: from dynamic.dsl.spammer.relay.thing...
From: admin at mydomain.com
To: valid.user at mydomain.com

and it's sailing right past spamassassin and confusing my mum, as she
keeps getting emails telling her account is to be terminated.

The From address can be a variey of things: webmaster@, accounts@ etc.

None of the spoof'd From addresses are actual valid accounts.

Is there a way to tell my exim (v3.35) or procmail (v3.22) or
spamassassin on Debian Woody to reject/direct mail with a From:
address that isn't a valid account on my domain to the spam bucket?

Ideally, somewhere globally that automatically knows about valid users
on the system so I don't have to maintain a seperate valid users list
for procmail/exim/sa?
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