On Wed, 2006-06-14 at 11:48 +1000, Paul Dwerryhouse wrote: > Greylisting relies on most spamming software being pretty dumb; some > only try to send a message once, and don't respect a "mailbox temporarily > unavailable" response from the SMTP server. A compliant SMTP client > would try to resend a message after a certain delay, when faced with > such a response.
Which is why it's so important for SMTP servers to guard against the possibility of being open relays. Certainly, greylisting can be circumvented by the evil doers simply injecting their messages to a legit nearby SMTP server, say, their ISPs which *will* properly retry. That, however, is easy for an organized ISP to track, hence those in the axis of evil writing SMTP clients that go direct to the server the target domain's MX record points at, which is why greylisting is a nice touch. AfC Sydney -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
