I usually Notepad the reply and set an iPhone timer to remember sending. It's enough to chill out.
Don't use it for people you care about. I've wrecked a potential date because he found out I've used Boomerang to fake indifference. It’s quite easy to figure out the email was sent through Boomerang and even how much delay youset. It's better to be "honest" or let it cool in Drafts.
As a first idea that popped into your head that's not so bad. You can practice at home first.
You can use GMail to set a filter for important messages.
Important messages should be handled shortly or you can get in trouble with your boss, bf, etc.
What about a possible federal regulation?