[G4] Attach mail message to outgoing mail

[C D T]

I've been trying for a long time - intermittently - to find out how to 
attach a mail message to an outgoing mail message.  Sometimes I am 
asked to "forward the email as an attachment".  Apple help has not been 
(helpful).  I have not seen the answer in any forums, Nor has anyone I 
have talked to known how.  So I thought I would ask you folks.

Attaching any other kind of file is not a problem.  Navigate to the 
file and click "choose".  I simply have not been able to find the mail 
messages anywhere to be able to attach them.  I'm not usually this 
dumb, but I've simply struck out repeatedly in my many attempts to 
figure this one out.

Apologies in advance if this is an inappropriate post for this forum.  
Thanks if there is an answer.

Dan