Maildir only suggests the safe delivery mechanism that gets around lack of atomicity guarantees in filesystems by using the guarantee that a rename is atomic as

Maildir only suggests the safe delivery mechanism that gets around lack of atomicity guarantees in filesystems by using the guarantee that a rename is atomic as a workaround. It suggests a way to add metadata about messages through the use of flags added to file names, but doesn’t specify what these must be. I don’t think, therefore, that Maildir as a standard provides enough to allow it to be relied upon for portability between clients (MUAs). That’s not to say a standard couldn’t emerge if there were some agreements.