[CentOS] Archive mail format?

Wed Nov 2 18:31:32 UTC 2011
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?

   Les Mikesell
     lesmikesell at gmail.com