This. I use Linux as my primary OS (with KDE) and my main complaint, by far, is the email/calendar situation. Mail.app simultaneously just works and gets out of my way, and I haven't seen a Linux email client come close to replicating that.

Every few years I convince myself I'll create a better email client for Linux, and I always start the project enthusiastically and stop soon after, when I get just far enough to be reminded of how complicated email is. Maybe someday I'll take a sabatical and actually do it...