Help with qmail editing to reject envelope forgeries

Hi,

I need a little brainpower to thwart the spammers from sending more mail 
to [email protected] which is an ezmlm list.

I've had the following in the staff/editors file for quite sometime and 
it is effective.

|/var/qmail/bin/bouncesaying '5813 rejected' /var/qmail/bin/except
/bin/sh -c 'echo "$SENDER" | /bin/grep -Eq "@yourdomain.com$"'

However, now the spammers are forging the SENDER id.  How could I add 
something to staff/editors to check the envelope "From " header, and if 
not from our domain, reject them?

Any other ideas?

Thank you,

dan