Postbox 2.5 now allows you to use Spotlight to search for Postbox messages by Topic.  For example, you type the following into the Finder search bar: "keyword:marketing" to find all messages that have the Topic "marketing."  You can then create a Saved Search and save it to your Finder sidebar.

To enable this functionality please perform the following steps:

• You must be running Postbox 2.5 or greater.
• The Postbox application must reside within your Applications folder.
• Within Preferences / General, the "Allow Spotlight to search messages" preference must be selected.
• Make sure that only ONE copy of Postbox (or Postbox Express) is on your hard drive or any mountable volume.  Use Spotlight to search for "Postbox" with Kind: Application to find any and all copies of the Postbox application.  If you have partitioned your drive to manage both Snow Leopard and Lion installs, only ONE copy of Postbox can exist (otherwise Spotlight might use the wrong importer).
• You must re-index your mail messages by going to the Tools menu, then select "Indexing" and then "Index All Folders..."  You may wish to perform this last step overnight to avoid work interruptions.