If you wish to reset your trial, you'll need to completely delete your profile directory, which will wipe out any previous settings that you had configured. Essentially you would be starting again with a fresh install.
IMPORTANT: If you are using POP or LOCAL accounts, you may have messages stored in folders that reside on your hard drive. If you delete your Postbox profile, these messages will also be deleted!
Locate your profile directories at:
Postbox 5 and below: ~/Library/Application Support/Postbox
Postbox 6 and higher: ~/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 Vista and Windows 7
Postbox 5 and below: Users\username\AppData\Roaming\Postbox
Postbox 6 and higher: Users\username\AppData\Roaming\PostboxApp
(Note: The AppData folder is folder is a hidden folder; to show hidden folders, open a Windows Explorer window and choose "Organize → Folder and Search Options → Folder Options → View (tab) → Show hidden files and folders".)
Next, delete your profile directory and then start Postbox again. This will reset your trial to 30 days.