Search currently just filters the available views, but there is not way to clear the filter again (except opening the search again...).
Either we only do a dedicated search view, in which case I guess this is irrelevant, or otherwise we need to somehow indicate that stuff is being filtered and there should be the possibility to clear the filter again.

I think we'll want filter in addition to search in the long run at least, as filtering is the only way to take everything that is on screen into account. (I have already selected my folder and am looking for a mail, now I just want to filter the list quickly, because otherwise I have to start over).