Specific steps will differ according to the mail program you are using.
In the Outlook web app:
- Open the folder containing the message to be attached.
- Create a new email message.
- Drag the message from the folder list into the new email message.
- Address the message and send as usual.
In the Outlook desktop client:
- Open the folder containing the message to be attached.
- Highlight the email in question.
- Press the Ctrl+Alt+F keys.
- Address the message and send as usual.
In Thunderbird:
- Open the folder containing the message to be attached.
- Create a new email message.
- In the Preview pane, click on the message and drag it into the body of a new message.
- Address the message and send as usual.