Use the optional mail
field in your user database to override the global mail_location
and use that to set the dbox format as an exception for the mailarchive account .
Edit: Since you're using LDAP you'll have to change the LDAP filter in Dovecot to include retrieval of an attribute to use as the mail field:
user_attrs = homeDirectory=home, uidNumber=uid, gidNumber=gid, mailLocation=mail
and populate the mailLocation
attribute for the mailarchive user in your LDAP user database with the correct value for a dbox:
mailLocation=sdbox:~/dbox
NB The mailLocation attribute used in the Dovecot docs is AFAIK not a default attribute in OpenLDAP but you can of course select any custom unused extension attribute or extend the schema with something custom.