spam relays on secondary MX

        there seem to be many interesting discussions on secondary MX and spam
        relays.  basically, secondary MXes normally do not check the existence
        of accounts (like foo(_at_)itojun(_dot_)org) and it will get queued, 
while primary
        MX can reject it at instant.  this may be ASRG topic, but i guess
        there's something to be done in SMTP level.

        some mailing list (do not remember which) even showed the attack ratio
        against secondary MX and primary MX, and secondary MX is attacked
        much frequently.

        
http://www.google.com/search?q=mx+secondary+spam&sourceid=mozilla-search&start=0&start=0&ie=utf-8&oe=utf-8

itojun