If you're like me, and if you're reading this there's at least a possibility you are, you don't keep your email at Gmail or in Thunderbird of Mail.app or wherever -- you keep it the way Ray Tomlinson intended -- on a server where you read it in plain text format. I use mutt with mboxes, but there are a lot of other possibilities. Anyway, after 10 or 20 (or more!) years of archiving your mail, it sometimes gets hard to find what you're looking for. Well, mairix is just the tool for you -- it incrementally indexes messages, and returns search results as a folder of messages for you to look it. It's free software, it's fast, and available in Ubuntu.