Not something that is wrong per se, but I moved away from the Mail.app because I switched to Inbox a few years ago because of it's productivity features. The way Inbox handled 'snoozing' mails (mails returing from snooze would appear at the top of the inbox) is not supported by Mail.app (or any other standard mail app for that matter, even Gmail at the time). So I switched to Inbox web app on the desktop as well.

I believe there are some plugins that support similar features, but I have not yet found a Mail.app snoozing that is portable as well (eg: Android/webapp support).