If you only see 2000 messages in a folder, you could be seeing our IMAP Limiting feature in action.
In versions 6 and higher, you can limit the number of messages that Postbox will fetch per folder. This feature is turned "ON" by default and is currently set at 2,000 messages (adjustable to 1,000, 2,000, 5,000, 10,000 or no limit).
You can increase these limits so see more messages in Postbox > Preferences (macOS) or Options (Windows) > Accounts > [your account] > Local Storage.