Quoting Bill Clark <[log in to unmask]>:

> I have 2 macs running Macmail, and I need the emails exported to Outlook.
> It appears there is nothing native to the program that allows me to do this
> -

(A) If the mail server is running IMAP (i.e. the messages are stored  
on the server), then any IMAP client (MacMail, Outlook, Outlook  
Express, Thunderbird ... ) can (even simultaneously) read the same  
messages. In this case, the mail is already "exported" and no further  
action needs to be taken.

(B) If the user has set up the particular mail client so that some of  
the messages are on the server and some are stored locally on their  
machine (quite commonly this is where sent, deleted, junk, etc, mail  
is stored. Local mail folders would often be the case for POP mail.).  
Logging into mail and dragging the local folders to the IMAP server  
will make them shared (exported), and we're back to the happier case  
of Step A.

That is, assuming that the user's email account can store all of the  
old messages :)