On Saturday, February 12, 2005, 2:34:20 AM, Alain Alain wrote:

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Generally speaking it may be better to apply this kind of filtering at the server level since there are economies of scale, especially in terms of things like DNS lookups and caching. If we suddenly get 100k more DNS clients, that could tax the name servers somewhat. If those same 100k users were using 100 servers instead, the DNS loading would be quite a bit less. In that sense centralization is desirable.

Mmmm isn't the dns server from the ISP caching the dns requests? I would think it doesn't make a big difference (except when a server is rsync'ing). The difference could be that end users check their e-mail not when arriving on the MTA, but later.

One difference is that the ISP's mail server may see many of the same spams within a short period of time, and the lookups would probably tend to be cached over that time span. Individual users may POP or IMAP their messages at any random time, so the DNS cache hit rate may be lower for them.

I think we're agreeing, but I've never tried to quantify the difference between these. We can propose that there's some difference but how much is unknown. I would propose a pretty strong cache effect for mail servers however.

Jeff C. -- "If it appears in hams, then don't list it."