Go into the e-mail that you wanted to "attach" to your new message. Hit "Forward". Then you can edit the old message, and add your new message before sending it.
I can't seem to figure out how to attach an old e-mail to one I want to send. I use Mail 2.1.3 on OS 10.4.9. If I wanted to attach any other type of file, I just hit attach and find the file. But because I'm already in my e-mail program, I can't find or figure out a way to attach an old e-mail to the one I want to send.

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