The ideal thing to do would be to think of an XML format for it (or find one) then get the mail out to some text format, parse it and spit it into a Database, then have it write out to the XML format, then write some XSL to display it. Those steps would be ok apart from the first because you’ll either need to get at the binary email or write some macro to loop through the mails in whichever client can display them, but if you could do it you need never worry about this issue again 🙂
Or you could import all of the accounts to a Gmail account…