Big Mail uses a technology called IMAP to connect to your mail provider.
To sync settings for Reply Later, The Bouncer and Scene categorisations, Big Mail uses a special folder (called “Big Mail”) to write data that can be read from all your devices.
Big Mail manages this folder automatically. It’ll periodically add, remove and combine these emails, so it’s best not to remove anything in this folder, otherwise you may loose some of your settings.
Deleting the folder will stop syncing Reply Later, The Bouncer and Scene categorisations across your devices. It won’t affect any emails in your inbox.