Excerpts from Carl Worth's message of Wed Dec 02 22:36:13 +0100 2009:
> As a side-note, I would recommend against making your auto-tagging
> scripts process only new messages. You can get a much more reliable
> setup by having your auto-tagging scripts apply to the global
> database. And this is not inefficient at all if you simply ensure that
> you only match messages which need the tag added.

I can see one clear case where that would become a problem: mistaggings.
If I set up something so that in 99% of the cases, it'd tag new mail
correctly with say "inbox unread mytag", I could quickly pick the
mistagged ones out and remove the offending tag. 

If you do it like above however, the autotag script would come merrily
along and retag them.
-- 
- Marten