Mia culpa to you, Sijmons et al. The solution I provided will not work the way I described.
I was talking with a friend about "archiving old mail" he runs a business and has tons of messages he wanted to archive. He had just told me about this solution he had found.
I didn't test this myself but took his word for it. I even questioned him about the emails opening the entire mailbox and could he read all the messages with the proper subjects? He said he could.
Finally at the end of the day when I could test it, I got the same results as others only numbers in Finder and only the chosen email would open.
What he was doing was overwriting his Mail folder on another drive running OS 10.4. Well, sure that'll work, but then you might as well set up for daily backup.
Oh well, again apologies to anyone I lead on a goose chase.
Good idea for a script Andreas!
-mj
macjack@gmail.com