Thanks for sharing a important etiquette for email.
But sharing or reminding this etiquette to the email sender’s before they send the message will benefit both the sender and receiver.
Formal Messenger ( https://www.formalapp.com ) is a email alternative app which has option to show etiquette guidelines like the one you shared here before they send the message.
Please check it out and your feedback is always welcomed.
Disclosure: I am a developer at Formal.