In the age of ubiquitous camera phones, there is no truly effective way to prevent the contents of a normal email message from being shared with others...

Some corporate email systems have integrated rights management features that limit who can see email message contents and what they can do with them. But I don't think that works outside your own organization.

There are or have been services that stores the actual message content on a website and verify who is accessing it through one means or another. When people send a message like that to me and force me to create yet another account I'm probably only going to use one time, I get very annoyed.

It is simply yes or no.

Let me ask the question another way: Does Thunderbird have a command that prevents an email recipient from simply using the "Forward" button to send that email on to a subsequent recipient? Some other email clients have this.

Such a command merely stops an immediate action to "Forward" an email, and, in doing so, sends the desire of the originator that he/she does not want the message sent on. It works very well among friendly colleagues.

As you have been told. No email system has such and option, except internal corporate mail systems. When they send messages to the internet their controls also fail completely. So the answer is NO

You might think you have such a button, but for mail on the internet there is no such function. Why, because there is nothing in the relevant RFC to allow such a limitation be placed on transmitted mail.

If you do not want correspondence to fall into the hands of someone other than those you approve, I suggest you refrain from committing the information to email. Or use attachments with a right management component in their parent applications. I understand Microsoft office documents have something like a rights management system, but I think it relies on the document being within a corporate network so the active directory can be interrogated to allow access.

Thank you! Both of you. The rapid responses were indeed delightful!