Heck, even if you add a confidentiality notice at the bottom, they can generally print it and share it.
Are we talking a legal work product? or are we talking about a personal email?
If you work for a government agency, for instance, and send a confidential email to related government agency partners (say, CPS sending a report to social services) and the recipient then shares it with unauthorized people the recipient that shares it MAY (depending on the law and the content of the message) be subject to criminal or workplace offenses.
But, simply placing a confidentiality notice on an email does not - by itself - magically grant it some grand protection. These boilerplate notices may protect someone from some civil liability in some instances, but as a general rule they are not legally binding on others.

