On Thu, Aug 13, 2009 at 08:11:54AM +0400, Cheney, Edward A SSG RES USAR USARC wrote: > > > In my limited understand, SMTP is a store-and-forward protocol. Will > > this affect your proposal? I imagine it would not be pleasing to > > click on something you expect to be interactive and have it respond 4 > > hours later. Maybe I should go read your draft more thoroughly -- > > you probably already thought of this. > > It is certainly possible an interactive reply from a distant mail server > could take 4 hours so liberal a time to live were allowed. I don't see > that as being common, however.
But it *is* common. One of the myriad anti-spam techniques in common use today is "greylisting", which can be implemented all kinds of different ways, but basically consists of giving a "try again later" response to first attempts to deliver mail messages. This works quite well against some spam delivery attempts (which are being performed by malware agents that can't/don't retry) and it works sometimes against some other spam delivery attempts (performed by mail servers that *do* retry) because, for instance, it buys time for DNSBLs to notice the spam source and list it. There all kinds of nuances to this, which I won't go into here because they're not relevant, but the bottom line is that lots of sites do this, including some large ones (e.g. Yahoo). The delays imposed vary, of course, but 1-4 hours seems to be ballpark. ---Rsk
