One of the big mistakes people make with email is to believe that what they are sending is only between the sender and the recipient.
The reality is if your email address is connected to your business or organization, than your business or organization owns your email address. So, if you work for The John Doe Company and your email address is email@example.com chances you, The John Doe Company has the rights to your emails. They can read them, they can delete them and they own them. This means that email you sent to a friend complaining about your boss has a good chance of being read by your boss.
Even if you have your own domain, or you use a gmail.com or yahoo.com account, you should never send an email thinking it’s truly private. While you can control who receives it initially (and even this isn’t always true as I have sent emails to the wrong address in my address book before), you can’t control what they do with that email once it’s received. If you sent an email to firstname.lastname@example.org. Understand Bob’s boss or others in the company might also have access to that account (whether Bob knows it or not). Even if no one else has access to that account, that doesn’t mean Bob might not forward it to someone you didn’t want it forwarded to.
As a general rule of thumb, never send anything via email that you wouldn’t want everyone to see. If it’s so private that it could get you in trouble if seen by others, don’t send it in the first place.
Have a great day!