I follow these rules religiously:
1) Never respond to work emails during the weekend, holidays and after 6 pm unless it’s an emergency (emergencies are very rare).
2) Never automate the forwarding of work-related messages to your personal inbox for any reason.
3) Immediately block/filter anyone who sends work-related messages to your personal email address.
4) Never check your email inbox when you wake up/before breakfast. Wait - at the very least - one hour.
5) Never check your inbox email after dinner and especially when you are already in bed.
6) Use the least amount of words possible in your messages, but be super clear and specific in order to avoid an endless back-and-forth. If an exchange is still ongoing after three messages, you are doing something wrong.
7) Never turn on email notifications (or app notifications in general) on your devices, except for very a limited amount of priority contacts.
8) Never respond to email messages more than once per day. Allocate a specific amount of time (no more than 30 minutes) to the task and do not exceed it even if there are still unanswered messages. Archive all messages that require "an automatic receipt" without opening them. Prioritize: important senders first or, better, only. When the time's up, close your email tab and do not go back to your inbox until the next day. "Inbox Zero" is a myth.
9) Not responding to an email message is a message in itself. Never respond to rude messages. If a sender doesn't take the time to write a decent piece of communication, what's the point of replying?
10) Mark as spam as many unsolicited, unwanted messages as possible to reduce clutter in your inbox. Never click the “unsubscribe” link in a newsletter or a promotional message: just mark it as spam.