I have 15,000 mails in GMail in various labels and some are imported from Outlook. Some mails don’t have labels. So how can I find all unlabeled mails in GMail?
You have to have some sort of other criteria to search for the emails other than labels. There's a lot of different search parameters that you can use listed in this post //www.makeuseof.com/tag/s.../With 15,000 emails it will be time consuming but at least it will be accurate.