- J. B. Nicholson:
What is the recommended file format for mail archives to share?
Maildir [1] has been around for decades and is widely supported. Other than mbox it stores one email message per file, so there is no problem identifying individual messages.
On my servers, incoming mail is delivered to Dovecot (configured to use maildir storage). Users can access via IMAP and Notmuch [2], the latter directly accessing maildir. Notmuch is what I am using right now. In Emacs. Because I can. ;-)
BTW, Notmuch does not care much about the directory structure, but attempts to parse and file not explicitly added to an ignore list as email. You can thus add files/directories not strictly adhering to the official structure and Notmuch will happily index them.
-Ralph
[1] https://en.wikipedia.org/wiki/Maildir [2] https://notmuchmail.org