Locking down secondary mx sendmail

[Doug Silver]

It seems the spammers like to target secondary mx machines and let those 
machines relay the email to the primary machine.  I've tried setting up the 
virtusertable file to at least reject invalid emails on the secondary mx 
machine, but it seems that sendmail bypasses that when it's doing mx relaying 
for that domain.  So, does anyone have some suggestions to partially lock 
down mx relay machines?

TIA

-doug