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While I'm not an MSU mail admin, I do believe the scenario you describe below works, and from my tests below, it doesn't even look like it has to be to/from the same users even. First connection from an IP address shows the following banner: 220 grey00.mail.msu.edu ESMTP spamd IP-based SPAM blocker; Wed Apr 4 18:06:43 2007 Subsequent connection attempts all show the same banner.. until around 7-8 minutes later, when the banner changes: 220 mx04.mail.msu.edu ESMTP Exim 4.63 Wed, 04 Apr 2007 18:14:52 -0400 At this point, it seems from/to tuples are completely ignored, and only frequency of IP address/range connections to MSU's mail servers is monitored. Perhaps someone from MSU's mail team could expand on that. -Russell > -----Original Message----- > From: MSU Network Administrators Group > [mailto:[log in to unmask]] On Behalf Of Chris Wolf > Sent: Wednesday, April 04, 2007 6:02 PM > To: [log in to unmask] > Subject: Re: [MSUNAG] Greylisting > > A user raised a question about MSU's anti-spam greylisting > that I couldn't answer. > > Suppose a user on a non-compliant email system sends a > message to a recipient at MSU. That message will get a > "temporary" refusal from MSU and then the non-compliant > server will fail to resend it, so the recipient will never receive it. > > But suppose the sending user sends another message (or even the same > message) to the same recipient within a short time. Will > MSU's system recognize this as the required "resend" and then > start accepting mail from that server? > > I've looked at descriptions of some implementations of > greylisting, and although they don't talk about this scenario > it seems as though it would satisfy some of them, which > apparently only look at three things, the sender's email > address, the recipient's email address, and the server IP. >