The creation of mbox / mdir is configured in main.cf with the parameter mail_spool_directory.
To configure mdir a / at the end of the path is required (same as for home_mailbox)
From main.cf
# DELIVERY TO MAILBOX
#
# The home_mailbox parameter specifies the optional pathname of a
# mailbox file relative to a user's home directory. The default
# mailbox file is /var/spool/mail/user or /var/mail/user. Specify
# "Maildir/" for qmail-style delivery (the / is required).
#
#home_mailbox = Mailbox
#home_mailbox = Maildir/
# The mail_spool_directory parameter specifies the directory where
# UNIX-style mailboxes are kept. The default setting depends on the
# system type.
#
mail_spool_directory = /var/mail/ #mdir format
#mail_spool_directory = /var/mail #mbox format
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