[CentOS] Archive mail format?

From: Les Mikesell <lesmikesell at gmail.com>

> I have a bunch of old mail spread variously across dovecot maildirs
> and mbox format files on several machines that are headed for the
> trash.  Is there anything considered to be a portable archive format
> for mail messages, and if so are there tools to copy into it - or do I
> have to pick a client and copy to its local storage?

Maybe as .eml files?
http://en.wikipedia.org/wiki/E-mail#Filename_extensions

JD