In desktop email clients, new messages arrive completely bereft of context. The only way to orient yourself is to either remember what the conversation was about or read through the mess of quoted text that may or may not be present at the bottom of the message, depending on what kind of email client or prefences the sender has. You could try searching to re-orient yourself, but good luck with that in Outlook or Mail.app.

Mail.app stores all of the meta-data for you email in a file called ~/Library/Mail/Envelope Index. You might wonder what the format of this file is… well it is a SQLite3 database. The contents are pretty easy to see, go to the terminal and type: