--- /dev/null
+=head1 NAME
+
+lei-mail-formats - description of mail formats supported by lei
+
+=head1 DESCRIPTION
+
+L<lei-q(1)> supports writing to several existing mail formats
+for interoperability with existing mail user agents (MUA);
+below is an overview of them to help users choose.
+
+=head1 Maildir
+
+The default output format when given a filesystem path, it supports
+parallel read-write access. Performance is acceptable for smaller
+directories, but degrades as mailboxes get larger. Speed and
+scalability are limited by kernel and filesystem performance
+due to the use of small files and large number of syscalls.
+
+See also: L<https://cr.yp.to/proto/maildir.html> and
+L<https://wiki2.dovecot.org/MailboxFormat/Maildir>
+
+=head1 Mbox family
+
+The mbox family consists of several incompatible formats.
+Locking for parallel access is supported, but may not be
+compatible across tools. With compression (e.g. L<gzip(1)>),
+they require the least amount of space while offering good
+read-only performance.
+
+Keyword updates (C<Status:> and/or C<X-Status:> headers)
+generally require rewriting the entire mbox.
+
+See also:
+L<https://www.loc.gov/preservation/digital/formats/fdd/fdd000383.shtml>,
+L<mbox(5)>
+
+=head2 mboxo
+
+The traditional BSD format. It quotes C<From > to C<E<gt>From >,
+but lines already beginning with C<E<gt>From > do not get quoted,
+thus automatic reversibility is not guaranteed. MUAs which favor
+L</mboxcl> or L</mboxcl2> may convert these automatically to their
+preferred format.
+
+Truncation is undetectable unless compressed with gzip or similar.
+
+=head2 mboxrd
+
+An evolution of L</mboxo>, but quotes C<From > lines prefixed
+with any number of C<E<gt>> characters and is thus fully
+reversible.
+
+This format is emitted by L<PublicInbox::WWW(3pm)> with gzip.
+It is supported by L<git-am(1)> since git 2.10.
+
+As with uncompressed L</mboxo>, uncompressed mboxrd are vulnerable
+to undetectable truncation.
+
+It gracefully degrades to being treated as L</mboxo> by MUAs
+unaware of the format as excessive C<E<gt>From > quoting is
+recognizable to humans.
+
+=head2 mboxcl
+
+L</mboxo> with a C<Content-Length:> header, C<From > lines
+remain quoted to retain readability with L</mboxo> and L</mboxrd> MUAs.
+However, it is easy to corrupt these files when using tools
+which are not aware of C<Content-Length:> and write out updates
+as L</mboxo>.
+
+L<mutt(1)> will convert L</mboxo> and L</mboxrd> to mboxcl upon opening.
+
+See also: L<https://www.jwz.org/doc/content-length.html>
+
+=head2 mboxcl2
+
+Like L</mboxcl>, but without C<From > any quoting. It is wholly
+incompatible with MUAs which only handle L</mboxo> and/or L</mboxrd>.
+This is format is generated by L<mutt(1)> when writing to a new
+mbox.
+
+=head1 MH
+
+Not yet supported, locking semantics (or lack thereof) appear to
+make it unsuitable for parallel access.
+
+=head1 IMAP
+
+Depending on the IMAP server software and configuration, IMAP
+servers may use any (or combination) of the aforementioned
+formats or a non-standard database backend.
+
+=head1 COPYRIGHT
+
+Copyright 2021 all contributors L<mailto:meta@public-inbox.org>
+
+License: AGPL-3.0+ L<http://www.gnu.org/licenses/agpl-3.0.txt>
+
+=head1 SEE ALSO
+
+L<lei(1)>, L<lei-q(1)>, L<lei-convert(1)>, L<lei-overview(7)>