This is not because of SaneBox. We do not deliver your email, delete it, or download it.
All SaneBox does is move Inbox email to either your SaneArchive folder (for very old email) or your SaneLater folder (for new email). SaneBox asks your email server to do the actual move.
The email being moved never leaves your email server.
If you are searching for a particular email and not finding it, you should look in your Spam folder. If it is not in there, you should try making your email search term simpler**.
So instead of searching for “red couch” you should try simply searching for “couch.”
If you often receive important email from new unknown contacts (i.e. through website forms or new sales leads), you also have the option to default SaneBox to keep email from unknown contacts in Inbox instead of SaneLater.