If you use Thunderbird, you have recorgnized for sure that Thunderbird, by default, just checks the INBOX of an IMAP account for new mail.

But you can configure Thunderbird to automatically check for new messages in all folders:

• Open your Thunderbird settings
• Open the Advanced tab
• Click on Change config
• Search for: mail.server.default.check_all_folders_for_new
• Set the value to True

That's it - Thunderbird will now automatically check for new messages in all folders.