You're very welcome! I'm afraid the developers were banking on "discoverability" of the user to find that one. I concur, and that would make sense that you thought something was buggy. I've been using Spark for my email client for over a year now. I opened the stock Mail app and played around with it, after seeing your post on here.
I like the look of Mail in dark mode. However, I think I really do miss the features that are beneficial in Spark, such as Snoozing mail, more swipe features, along with scheduling an email. I still don't understand how Apple hasn't implemented this in their own stock app. It's nice to compose a message to get it off your mind, no matter the time of day, but then schedule it for the next morning as to not look like a complete insomniac.