How to prevent an email from accidentally being forwarded?

It's not possible to prevent forwarding. If you have sensitive content, it should be locked behind credentials. Force the user to login to view the content.

You can't prevent an e-mail from being forwarded. The receiver will have a copy of the data the email consists of. He can do with it whatever he/she wants.

Securing the data with credentials is not really effective. If you share the credentials with the receiver along with the sensitive data, who says he/she wont share or forward the credentials/key/etc.?

If you can't trust him to not forward the data, you can't trust him with the credentials either.

If you are in control of a Corporate network, and can control the email clients receiving the emails, then you might want to consider Information Rights Management. You'd have to have all the email clients running Outlook (2003 or above), but it does seem to prevent data being forwarded, copied, printed etc.

I'd only use it as a warning feature though (i.e. to stop someone accidentally forwarding emails). It's not going to be hacker proof.

Use something like BigString

http://www.bigstring.com/info/features.php

• Self-Destructing Email
• Recallable/Erasable Email
• Non-Forwardable Email
• Non-Printable/Savable Email Advanced
• Email Tracking

Its free :D

No such thing exists. You can't stop people from forwarding emails; it's just data.