If you wish to reset your trial, you'll need to completely delete your "PostboxApp" data folders, which will delete out any previous mail databases and settings that you had configured.
IMPORTANT: If you are using POP or LOCAL accounts, you may have older messages stored in folders that reside on your hard drive. If you reset your trial, those messages will be deleted.
1) Locate your Postbox data folders 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.)
Postbox 5 and below: Users\username\AppData\Roaming\Postbox
Postbox 6 and higher: Users\username\AppData\Roaming\PostboxApp
(Note: The AppData folder is hidden. 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".)
2) Next, delete the Postbox or PostboxApp data folders, then start Postbox.
Your trial will be reset to 30 days.