If many people share that opinion, many will forward a particular message, and they will forward it to many other people. Most email programs insert the headers including To: and often Cc: by default when you forward a message.
What Happens if You Do Not Remove Addresses
This has the single advantage that everybody knows who already got a particular message. No need to forward the same mail to the same person twice.
But including all header information with all other To: and Cc: recipients has overwhelming disadvantages.
- The message looks horrible, and often the actual content is buried and hard to find between all the email address.
- You share the email addresses with third parties without the consent of the addresses' owners. I wouldn't want my address given to I don't know who just because somebody forwarded me something that she also happened to forward to 150 other people.
Remove All Email Addresses When You Forward a Message
That's why you should always
- remove all email addresses
(except for the original sender if you deem it appropriate) before you send a forward. If you forward the message inline, just highlight them and hit Del. If you forward the message as an attachment, you might have to take some extra steps.