[Dovecot] Keyword Flags

I mostly use Mutt to read mail but today I fired up Thunderbird.

I have a shell script that looks for new mail, which looks for files in cur/ and new/ ending with the server hostname or a comma.

When I fired up TB it seems that all new mail has been given a flag of ,a which I gather means it matches a keyword.

My questions are:

How did this happen? Did TB do something strange to my mail when it fetched headers? If I change my search parameters to ',[a-z]' will this only catch new mail?

Cheers

-- When all you have is a Swiss Army Knife, every problem looks like email. -- Peter da Silva