Basically, maildir format is a set of directories with the layout
Maildir/ cur/ new/ tmp/
Each message is stored in a separate file in either the cur or new
directories, depending on whether or not it has been seen by the mail
client. Maildir folders do not need file locking (no ':0:' in
.procmailrc). The tmp dir is used internally when writing new messages
to the folder.
You do not have to configure anything to get Mutt to read maildir format
mailboxes. It can determine the mailbox type automatically by inspecting
the directory structure.
To create a maildir format mailbox, either:
mbox_type="maildir" and create a new folder in Mutt