Greylisting bug with Exchange

Earlier this week the boss forwarded some bounced emails to me and asked me to figure out what had gone wrong. The weird thing was that the email was being greylisted, so it shouldn't have bounced:

This is the Symantec Mail Security program at host

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to <postmaster>

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                    The Symantec Mail Security program


<>: host said: 451
<>: Recipient address rejected: Please
try sending again. (in reply to RCPT TO command)

Turns out that Symantec Mail Security is meant to sit in front of an Exchange server, and it turns out that Exchange has a bug (or had; I'm unsure if it's been fixed) where doesn't requeue email that's been greylisted, and later on bounces it back to the sender without ever having retried.

From what I can tell, is run by, which provides high-speed access for hotels…so I'm probably not the only one being asked to explain this bug. :-)