Postbox is a Thunderbird-based mailer for Mac OS X and Windows that has some Xobni-like search features and Mac OS X Address Book integration:

1.0 beta is not available for Linux, even though it might well trounce all other existing mail applications. My take?

  1. It’s going to become a paid product, and nobody on Linux would actually pay for software.
  2. It uses .NET to an extensive degree, which complicates matters somewhat.

It also displays varying degrees of missing functionality (one of the most irking ones is the inability to drop files on the dock icon for starting a new message) and slow indexing.