Doing the usual Monday morning thing I saw that IIS SMTP had some messages sitting in the queue that were a bit old. I did a little digging in the log files and saw that as with other instances it had to do with the receiving server on outgoing mail returning a 451 code. This is a pretty common spam-fighting method (one I'd probably use myself if Mercury supported it). It seems to give IIS fits, though. IIS seems to attempt the initial delivery, but retries are never done. The log file doesn't indicate that the QUIT command was sent and I wonder if that has something to do with it. Maybe IIS never realizes the connection is cut and so never retries.
I should report this to MS, but I don't know that it's really worth it. For the time being I've unchecked "Attempt direct delivery before sending to smart host" in order to avoid the problem in the future. I'll need to shut down the service, though, so I can clear out those older messages.
