Or how much I can ephasize it... I've been delivering this message in every privacy and security training class I've delivered for over 10 years....
Never put private information, or send communications over the internet, unless it is in a secure, encrypted, controlled, and verified way; and in all steps of the end to end chain from senders eyeball to receivers eyeball, it stays that way. note: If you actually want email privacy, I recommend PGP for starters.
Email, is almost exactly the opposite of that. The only way email could be any less private is if it automatically remailed itself out to hundreds of people, who might then forward it on to others.
I must have delivered this line 10,000 times by now... Never put anything in an email you wouldn't want published on the front page of the Washington Post....