This has the huge advantage of keeping the original message intact (the disadvantage is that you're probably revealing all the recipient's addresses; you can avoid this is with a few extra steps).
To forward a message as an attachment in Evolution:
- Highlight the message you want to forward in its Evolution folder.
- Select Actions | Forward | Attached from the menu.
- If forwarding as an attachment is your default forward style in Evolution, you can also use the Forward toolbar button.
- Address and edit the message as usual.
- Click Send to deliver it.
If you prefer forwarding messages inline, Evolution can do that, too.

