Some time recently when this question was asked, we found that the "Auto:" is hardcoded and there didn't appear to be any provision to allow the user to alter it. Additionally, the inclusion of "auto" is in line with the recommendations of the relevant RFC.

If you desperately want to change it, I may be able to find that discussion and point you to the suggested hack. I don't recall if it involved recompliling the source; a lot of Thunderbird is implemented in javascript and these parts are quite trivial to alter.