I wondered that, but as Postbox is Thunderbird on Steroids the core file formats are probably the same, certainly close enough for drag and drop migration:
Going back to Thunderbird:
http://www.postbox-inc.com/support/exporting_to_thunderbird
Going Back to Apple Mail:
http://www.postbox-inc.com/support/exporting_to_apple_mail
The only issue I've had is the twitter integration doesn't work, but that's not something I'm bothered about. This is a known issue due to be solved in v3.0.9. This version is however several months late as it was promised in November.
Everything on my Thunderbird annoyance list is implemented or fixed in Postbox and it has core support for almost all of the add-ins that I use.
The only gotcha might be if you use an unsupported Thunderbird add-in as they have to be updated to allow use in Postbox.