Make a backup of your 'Postbox Prefs' folder (or whatever you named it).

Exit from Postbox on the first computer.

Decide which computer has the most accurate or current version of your Topics, Responses, Signatures, and Address Book data, then save this data from this computer first to the sharing service directory of your choice, such as 'Postbox Prefs.'.

If you have multiple computers and wish to store this data within a syncing service such as Dropbox:

Within Preferences / General, you can specify where you want these preferences stored.

Since Postbox preferences are written to disk when you exit from Postbox if you decide to store this data on a syncing service, we recommend exiting from Postbox from one computer before using it on another. The capabilities of this approach are dependent on the syncing service itself. While we do not support syncing of this data per se, theoretically you can select a folder within Dropbox or Box, and then have this data synchronized across multiple Postbox installs, for example, home and work computers. In Postbox, you can store Topics, Responses, Signatures, Quick Posts, Themes, and Postbox Address Book data in the location of your choice.

Dropbox not syncing between compters