All Postbox settings and data are stored in a data folder that resides outside of the application. You can transfer your Postbox data folder from one machine to another by following these instructions:

• Make sure that Postbox is installed on both the source and target machines. 
• Quit or exit Postbox on both machines.
• Next, move the Postbox data folder from the source machine to the corresponding location on the target machine.

The location of the Postbox data folder is as follows:

macOS
Postbox 5 and below: ~your home folder/Library/Application Support/Postbox
Postbox 6 and above: ~your home folder/Library/Application Support/PostboxApp

(Note: In macOS the Library folder is hidden. To access it, open the Finder, hold the Option key, then select Library from the Go menu.) 

Windows
Postbox 5 and below: Users\username\AppData\Roaming\Postbox
Postbox 6 and above: Users\username\AppData\Roaming\PostboxApp

(Note: The AppData folder is folder is a hidden folder; to show hidden folders:

• Windows Vista / 7 - Open a Windows Explorer window and choose "Organize → Folder and Search Options → Folder Options → View (tab) → Show hidden files and folders".
• Windows 8 and higher: Open a Windows Explorer window and choose "View Tab → Options → View Tab → Show hidden files and folders, then click the Apply button".)