How do I disable BadMail usage for SMTP service in IIS6.0?

I have a relay server in front of Exchange 2003 running antispam and
antivirus. It is a windows 2003 server box with IIS SMTP installed. I
noticed that the BadMail directory builds up really fast and would like to
disable this feature. I've found docs on how to do this in Exchange 2003 but
not for IIS SMTP service. Does it use the same registry keys as an Exchange
SMTP virtual server? If not, is there instruction somewhere on how to
perform this?

thanks