Here are directions to create an "AutoReply" done right in FirstClass. This is not completely "right" in my book, but it is better than any other solution for the time being.
First, a brief explaination of why it is REQUIRED that a rule this complex be configured when setting up an "Autoreply" rule.
Consider this scenario:
1) You setup an autoreply rule on your mailbox that says, in general terms: "Every time you receive a new message, send a reply to the sender than says: 'I am out of the office for a while. Sorry.' "
2) "User b" user does the same thing.
3) "User C" addresses a message to you and "user b."
4) Your mailbox sends user B an "away" automatically generated message, and s/he sends you one.
5) On reciept of the auto reply from User b to you mailbox, your mailbox creates another message to user b which causes user b's account to generate a reply. This ping pong of auto generated messages will continue until the mailboxes fill up.
That is bad. Note that the above is only one scenario that can cause a mail loop disaster or "Mail-Storm." There are many others. Regardless of the exact scenario, mail loops are bad. The following rule will result in a these loops being stoped in most cases. This solution does not result in only one message reply per email address. However, it is a step in the correct direction.
We are all waiting for the developers at FirstClass to add an option that will allow for such an autoreply to be sent to any given email address only once per xx days. This feature is planned, but in the mean time, this will have to do.

NOTE: If you have any questions about this setup, please contact us. Mail loops cause us untold grief frequently, and we would far prefer to help you setup a rule that will help prevent a loop, than be forced to work to sort out the cause of one, and shut it down.