I've been using Spark since the very beginning. I was an original waitlist-starting user of Mailbox, and was excited to find spark almost immediately after Mailbox went extinct. This is the best email experience on every platform, and has a variety of productivity features that make it my go-to.
Ever since Mailbox became a graveyard app, I have been looking for a worthy replacement – and Spark is as close as it gets! The UI is clean, combining dark and light colors which helps with using the app; the side menu being dark while the rest of the UI has a white background makes you focus on the important stuff (the mail list and the mail in focus)!