In short: no.
And it can get even worse. Much worse.
Once you’ve sent email, it’s gone and you lose all control over it. And by all control, I mean:
Note I said “reliably” in a couple of cases. There are technologies, such as “bugs” or simple image tracking for HTML formatted email, that can sometimes allow you to tell that an email has been opened. This is not 100% accurate, and hence cannot be absolutely relied on, mostly because modern mail clients often have the required functionality turned off by default as a security and privacy measure. And “opening” an email does not necessarily mean that the email was actually read or even seen by anyone.
But in regards to your question, it’s the last one that should cause us all to think.
I’ll put it more clearly: you cannot prevent someone from forwarding your email, and you also can’t prevent them from changing it before they do so.
Get that? Not only can they forward your email, but they can change what your email says. When someone forwards an email, that email simply becomes the body of a new email, that can be edited before hitting send. And by edited, it could be as simple as changing a “yes” to a “no”, an “I love you” to a “get out of my life”, or perhaps “the boss is a genius” to “the boss is an idiot”.
So, if email is such an unreliable medium, what can you do? Well, the simplest first step for this particular issue is to save all your outgoing email. That way you’ll have a record of what you really said. Second, don’t say anything in email that you wouldn’t want to be made public, and when things get “close”, make sure you’re only emailing it to someone you really trust.
There are technologies such as cryptographic message signing that will allow message tampering to be detected, but currently they’re used mostly only by some businesses and the very tech savvy. To be honest it shocks me how much sensitive information is transmitted in email without any protection whatsoever (other than the silly disclaimer that boils down to “if this message wasn’t intended for you, forget everything you just read”).
I’m hopeful that over time these types of security solutions will get easier, and more common.
And the bottom line is that if someone can read your email, they can still forward it, or copy/paste it to another email, and send it to anyone they choose, one way or another.