M2dir aims to provide a standardized way to store a collection of email messages as files. It is similar to Maildir/Maildir++, but aims to be simpler and more thoroughly specified.
The canonical home of the m2dir project is at sr.ht/~bitfehler/m2dir.
The current specification can be found at man.sr.ht/~bitfehler/m2dir.
TBD.
If you have questions about the project or the specification, want to contribute, or add an implementation to the list: please send an email to the mailing list.