Re: IMAP extensions needed for SPAM/HAM and WHITE/BLACK listing

[Iljitsch van Beijnum <iljitsch@xxxxxxxxx>]

On 6 jul 2009, at 10:24, Dave Cridland wrote:

> > But if you want this, I'd say that it needs to be a fractional  
> > thing,  not a binary spam/no spam indication. For instance, the  
> > server could  give something a spam score of 2 and the client also  
> > 2 and together  that would be 4 so the message is presumed to be  
> > spam (assuming the  spam threshold is 3), but in a binary system no  
> > spam OR no spam = no  spam.

> The notion of using two spam filters in concert to attempt to make  
> an overall decision is basically flawed, because it fits into one or  
> other case above - if you do go to the effort of having a range  
> based spamminess from one, you'd have to use it as mere input into  
> the other's decision process, since combining them naïvely would  
> produce poorly weighted results.

Right. The filtering would have to be complimentary. So perhaps there  
should be an indication of the type of filtering done, too. The reason  
to combine server and client side approaches is that servers can  
easily do blacklists, and running any type of filtering on the server  
gives it the opportunity to reject the message during the SMTP  
session, creating a good error message for real senders but no useless  
bounces to innocent third parties in the case of spoofed senders. On  
the client is useful because the client can typically better afford to  
run CPU intensive stuff like baysian filters and the client probably  
has a better list of previous correspondents that it can use to  
whitelist.