For once it's not about spies and bots from the cyber world. It can happen within your own email circle.
If you are eager to prevent recipients of your emails from editing the text of your work, what can you do about it? Unfortunately, just about nothing!
If you're afraid of being misinterpreted and you have a hunch the recipient might be up to some trickery, try not to share such relevant contents via emails as email forwards can easily be modified, a distinct downside we didn't have to fear until some years ago.
You cannot prevent others from forwarding your emails which is fine. But you also cannot stop them from changing it or doing pretty much whatever they want with it. If any of the recipients in your email list has no qualms modifying or deleting portions of your message / work they consider their entitlement, that could result in gross misinterpretation of your views and misrepresentation of yourself, and thwart your efforts in circulating a precise ideology.
Once you send an email, you lose all control over it. When someone forwards your email, that email simply becomes the body of a new message that can be edited before hitting the 'send' button. And editing could be as easy as altering "yes" to "no", “like” to "hate", "a genius" to "an idiot" and more. Entire paragraphs can be deleted, in some cases rewritten, and the essence of your original message could be completely lost after a single forward!
If you were worried about your email being forwarded, this bit should add to your worries. You need to absolutely trust your recipients.
What can one do to overcome the risk? Save all your outgoing original stuff. That way you’ll have a record of what you really typed or pasted. If a misunderstanding arises because of someone interfering with your forwards, you could take a screenshot of the portion modified from the original to catch the culprit.
If you're emailing articles, stories or serious stuff you want to be forwarded but don't want the forwarders to tamper with the contents, simply send the link of the original source with a brief comment. Sending the source link along with the work is pointless. The tamperer can remove the link and modify your work and the recipient or recipients of the forward will never know about it.
Infuriating, but people do such things.