The best place to do this is via your webmail account using a browser. Many offer these types of responses.

It can be done via Thunderbird, but you would need to leave computer logged on with access to internet and Thunderbird running. The computer must not go to sleep or switch off. Anyone who has access to the computer could have access to your email account. Hence this is not advised for security reasons. But you can create a new Write email saying you are not available and save it as a template, then set up a Message Filter ; if 'from' is in address book then reply with template.