A common reason to forward messages as an attachment is to report suspected phishing email.  Forwarding as an attachment preserves helpful diagnostic information, which is lost when simply forwarding the email.

Outlook Client (Windows)

1. In the "Message List," select the message you wish to forward.
2. Press the Ctrl, Alt, and F keys, simultaneously.  A new message will open with an attached message.
3. Enter relevant information in the "To," "Subject," and "Body" of the message.
4. Click "Send."

Outlook Client (Mac)

1. In the "Message List," select the message you wish to forward.
2. Right click on the message (Press and hold the "Control" button while clicking).  A menu will open.
3. Click on "Forward As Attachment." 
4. A new message will open with the selected message as an attachment.
5. Enter relevant information in the "To," "Subject," and "Body" of the message.
6. Click "Send."