A very brief tech mystery today!

We have not upgraded to Exchange 2010 yet at work and are still on Exchange 2003 (yes, 2003).  That upgrade is scheduled for a little later this year, but until then, I have to continue to support 2003.

I ran into a strange mail flow issue today that ends up being the result of a combination of Exchange 2003 and Outlook 2007 or newer.  One of my users was trying to send an update to a meeting request, but recipients outside of our domain (external SMTP recipients) were getting stuck in the queue.  The additional information in the queue just said "Unable to open the message for delivery."  That clued me in that it wasn't an external SMTP rejection of the message (invalid email address or whatever).  So I did some googling.

It turns out there's actually a known issue for Outlook 2007 or Outlook 2010 (or newer) clients sending meeting updates via Exchange 2003.  It must not happen all the time, because we've had Outlook 2007 deployed for years and never saw this issue before.  In fact, it didn't start happening until a user on Outlook 2010 sent a meeting update.  Maybe it's just coincidence – I'm not really sure.

Anyway, here's the KB with all the details.  The article isn't clear, but to double check if you have this error by enabling diagnostic logging, you need to set MSExchangeTransport\Exchange Store Driver to Maximum.  Once you do that, go back to the queue, select the outbound server the stuck message is sitting in, right click and select "Force Connection."  Then you can look in the event viewer for the specific error to confirm this is your issue.

Don't forget to set the registry entry after applying the hotfix!