To forward one or more emails exactly as they were sent, you can 'forward as attachment'. Here are the methods to do this for various types of email software. This is particularly useful when forwarding email for technical reasons, for example any spam that has been received.
Thunderbird (including Netscape, SeaMonkey)
Select the message or messages you want to forward. (You can select multiple messages by holding the 'Shift' or 'Ctrl' keys when clicking.) Then from the menu bar at the top of the screen, choose 'Message' > 'Forward As…' > 'Attachment'. The message should appear in the attchment box in a new message. Type the destination email address and press 'send'.
Outlook Express / Windows Mail
Right-click the message that you want to forward. In the context menu that appears, click 'Forward as Attachment' (this is also available from the 'Message' menu) at the top of the screen. Type the destination email address and press 'send'.
If you want to send multiple messages, you can also drag and drop messages onto the message window. You can also select multiple messages by holding the 'Shift' or 'Ctrl' keys when clicking, and then right-click and 'Forward as Attachment' as above.
In Outlook 2007, 'Forward as Attachment' is available from the 'Actions' menu when one or more emails are selected.
To forward multiple emails, select the messages (by holding the 'Shift' or 'Ctrl' keys when clicking). Then right-click and select 'Forward items'. This automatically attaches the emails to a blank message – Outlook decides for itself whether to forward as an attachment depending on how many messages you are forwarding. Type the destination email address and press 'send'. (You could also use this to send a single email, by selecting a message you don't want to send, and then removing that attachment before sending.)
Alternatively, start a new email message. You can then either drag and drop one or more messages into the new message (if after selecting the message you can't see the new message window any more, drag it instead to the taskbar button for the new message and the window you want should come to the foreground); or from the menu, click the 'Insert' menu and then choose 'Item'. In the top half of the window that opens, select the folder that contains the email you want to send, usually Inbox. Then select the message you want to attach, and make sure that any 'Insert As' option is set to 'Attachment' before clicking 'OK'. Type the destination email address and press 'send'.
You can tell Outlook to always forward as attachment by going to 'Tools' > 'Options' > 'Preferences' > 'E-mail Options' > 'When forwarding a message', and selecting 'Attach original message' > 'OK'.
Open the message. Click on the button to show the full headers, then do File > Save As. Once you've saved the message, start a new email, and attach the file you have just saved.
Open the message you want to forward. Under the 'View' menu, select 'Message' > 'Long headers' (for OS X 10.2, Jaguar it is 'View' > 'Show all headers'.) Then click 'Forward', type the destination address and send. Alternatively, the drag and drop methods above should work.
Select the message(s) that you want to forward as attachment, then click on 'Message' -> 'forward as' -> select 'Attached'