vi /etc/exim4/exim4.conf

local_delivery:
...
  directory = /var/mail/${lc:${domain_data}}/${lc:${local_part_data}}/INBOX
...

vi /etc/dovecot/dovecot.conf

...
#mail_location = maildir:/var/mail/%Ld/%Ln:UTF-8:INBOX=/var/mail/%Ld/%Ln/INBOX
mail_location = maildir:/var/mail/%Ld/%Ln:INBOX=/var/mail/%Ld/%Ln/INBOX
...