Use notepad to compose a message and save to "my documents"
In OE - Message - Create rule from message -
1. conditions - scroll down and tick "for all messages"
2. actions for your rule - scroll down and tick "reply with message".
3. Rule descrition - click message and open the message you created in notepad - click OK
when you nolonger require to auto reply just click - tools message rules and remove the rule.