Don’t give up if you lament the lack of advanced functionality in the default Mail app on Mac, as there are a number of third-party plugins available which add features to Mail. One of the more comprehensive plugins is Mailbutler, a tool that lets you snooze messages, schedule messages to be sent later, attach notes to messages, set follow-up reminders on sent messages, and more.
Mailbutler works not only with the Mail app on Mac, but also with Gmail if you use Chrome as your browser. I’m using Mailbutler with the Mail app on Mac. Although the Gmail version appears to work similarly in my limited testing, this review is written from the perspective of an macOS Mail user.