We have a support center that is not password protected for our clients (something we plan on implementing in the future). We do not want anyone who is not a client to access this site, so we are hesitant to link to it directly in emails because of the chance the email might be forwarded to a non-client. Is there anyway to disable links within the email HTML when it is forwarded?
Not sure if this could be an option for you, but...
We use the "forward to a friend" token in emails similar to what you're describing. Although it doesn't prevent non-clients from accessing the information, it does allow you to obtain contact information from the person who received the forwarded email. This way you're able to track who else is consuming the content.
Just from a technical perspective, there is no way to achieve this. Links are hard-coded into emails as they are sent, so the link is forwarded along with message. I don't think there's much you can do here other than implementing password protection.
I guess you could do it quick and dirty, if you were referencing something sensitive. After the email is clicked and the page is loaded you could wipe the data being displayed. Kind of like a self-destruct function.
Clicks link in email & visits web page -> wait 1 minute -> change data value = null
Doesn't work if the email is forwarded before it's clicked by the intended recipient, though.