Is there a way to prevent Postbox from indexing certain mailboxes?

Is there a way to stop Postbox from indexing certain folders when searching? In Apple Mail, for example, you just add the mbox file to the Spotlight exclusion list.

The inability to do this means that the same message appears in search results as being in the Inbox and in All Mail, or in Sent and in All Mail, etc. I realise this is related to the way Gmail handles IMAP, but the ability to stop indexing some folders would fix it.

Sorry if this has already been answered.