So here's a quick pro tip for Apple's Mail.app. I often need to reference emails that people in my team have corresponded on or emails that I've sent to them. I don't segregate emails into folders rather I have a "completed" folder where everything goes once I've acted on it. This is easier than having a large folder structure for every conceivable topic, customer or co-worker. The search app is robust enough that I can usually find what I'm looking for quickly. This is specifically more helpful when using an iOS device.
When a need to find an email from a teammate quickly, rather than search in the completed folder under their name I've developed smart mailboxes for them. This includes email in the sent folder so when I click on the smart mailbox it brings up a chronological history of my email correspondence with that person. I've listed some screen grabs below for reference.