It looks like Postbox is opening their support for third party add-ons again.

Labs

Similar to add-ons, our Labs feature will allow for experimental 3rd-party code to be used within Postbox.

The first Labs project will be OpenPGP support provided by a specially designed version of Enigmail for Postbox and can be enabled under the Tools menu (remember to add the appropriate buttons to the Compose window toolbar).

Unlike add-ons which can be arbitrarily installed, only a select number of Labs projects will be available for Postbox. Given that Labs projects contain 3rd-party experimental code, they are not officially supported by Postbox, Inc.

If there's a 3rd-party solution you would like to see in Postbox, contact the 3rd-party developer directly. Please do not contact Postbox asking us to support code we do not develop, thanks!