Look in Tools>Message Filters and you probably have a filter set there to do that. Just turn it off.

Nope nothing in there, But thanks for trying

Did you do it in Thunderbird or on your providers web page? those type of out of office replies are not really supported in Thunderbird, except as a message filter and even then Thunderbird must be running and receiving the mail to send the reply. Most email providers offer such on their web site and that is what the relevant support article recommends.