MH is a suite of shell commands which implement a mail user client directly in the filesystem. All received mail exists as individual files in directories, one per mail folder. Each filename is a number, defining the sequence of incorperation into that folder. messages can be tagged inside the file, or collected into sequences inside each folder.
Each MH command is a distinct UNIX command, implementing one function.
The procmail system understands the MH file/directory paradigm, and can save incoming email as individual files by use of the correct syntax to specify the target is a directory.