I guess you are all familiar with the Out of Office Assistant in Exchange server. Whenever you activate it Exchange will send a messages to anyone who sends you mail that you are out of the office. I never really thought much about the internals of OOF Assistant, but a question from a customer prompted me to do some digging. The question was simply about how often the OOF Assistant would send messages, or Out of Office replies, as they are called. I had no idea and had just assumed that OOF Assistant would send a reply for every message received, but a quick test revealed that that was not the case. It turns out that OOF Assistant maintains a table of all senders sending you mail when the OOF is activated. It will only send an OOF message once to each sender. So if I am away for two weeks and you send me 100 messages (from the same account) you will only receive one OOF messages from me. Neat! More info about this is in this KB article:

Only one reply is sent to each sender when the Out of Office Assistant is enabled (http://support.microsoft.com/kb/157961)