A very simple one, pretty much the same as to car alarms.
Turn it off.
Outlook Express also isn't the best email client - certainly in terms of security - and there is a great free alternative made by the same nice people who made the Firefox browser.
It's called Thunderbird :
http://www.mozilla.org/products/thunderbird/
Unless your autoresponder has a specific setting to not send repeated replies to the same address within a specified time frame, there is no way of avoiding this problem because the software is doing what you told it to do - monkey see, monkey do.
It's much worse on mailing lists, as all it takes is one person with an autoresponder and their client sends replies to the entire list as soon as they get one message, which they get a copy of and duly autorespond to the list with their polite "out of office" message, which is posted to the list etc etc and an entire body of users are spammed until either inboxes get filled up to capacity, everyone has unsubscribed or the offender has been removed or shot.
You can get autoresponder software but in order to find it you will find yourself in the grim world of mass email marketing "entrepreneurs".
I read somewhere that autoresponse should be handled by ISPs so that users who have gone on vacation shouldn't have to leave their computers on 24/7, which you would have to do with your current setup.
wangi?