You dont need an auto responder for such a system.
Why not try this.
You know which combo is selected and instead of sending an email to your addresses why no send it to your same address but also a preformated reply to the customers address that you are sending as automated one. It has an added benefit
1. You dont have to have multiple email addresses to maintain (Delete mails from each account and stuff like that)
2. Users each time getting reply from different email address on different queries will trigger alarms on his side as to what is happening and sometimes it may go to spam folder as well. If you have one email address and if user has that added in his address book chances are less for such mishaps