You can clear the cache by tapping More... | Global | Advanced Features | Clear Cache.
After the process is done, BlueMail will re-download your emails according to the amount of days set in your Days to Sync setting.
Android Your stored Emails/Task/Later/Done will be cleared. Your Settings and Accounts will be kept intact.
Desktop: Clearing app data will permanently delete all of your local BlueMail data, including your accounts, settings, and local emails.