I originally set my Exchange Server 5.5 Internet Mail Service (IMS) machine to Inbound only. I recently changed that IMS to Inbound and outbound mode and was surprised to see several thousand messages flowing out from it. What caused that behavior, and how can I keep it from happening again?

This behavior occurs because the Message Transfer Agent (MTA) can't tell that the IMS is set to Inbound only. In some circumstances, the MTA routes messages to the IMS queue on a server that can't process outbound messages. The messages then sit there, silently waiting for the IMS to process them. Unless someone complains that mail is missing, you probably won't think to check the IMS to see whether any messages are in that queue. Switching the IMS to a mode in which it can process outbound messages will flush that queue and send all the mail (even if it's very old) on its way. To avoid that problem, either don't change the IMS mode or switch to Exchange 2000 Server.