On Thu, Aug 13, 2009 at 08:11:54AM +0400, Cheney, Edward A SSG RES USAR USARC
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.