I have a legacy email system with many unix mailbox files populated by fetchmail and procmail for some mailing list subscriptions. These live on a mail server which has NFS and Samba so it can export my home directory (for example). Mutt works well with these files but really doesn't work well with HTML formatted email.
What I would like to do (if possible) is simply get another MUA that will work directly on the mailbox files but will render HTML email. Bonus points if I can use it from both Linux and Windows.
At one point I did experiment with WU-IMAPD (which can sit directly on mailbox files) but the user agent insisted on maintaining its own folders instead of saving to folders on the server. Thus, I'm really looking for something that will read and write to mailbox files directly.
So, any recommendations for a mail user agent that can (a) read and work with a large collection of unix mailbox files and (b) is preferrably portable?