I met the same situation when I use iredmail to install a mail server.
As Zhang said, iredmail does not use online config, ie. slapd.d to store configurations.
It uses slapd.conf as a static configuration file.
All these two files located at /etc/openldap/slapd.d (this one is a directory) and /etc/openldap/slapd.conf (file)

According the official document of openldap, you can change the static configuration to dynamic.
Follow the instruction in,
https://www.openldap.org/doc/admin24/slapdconf2.html
Section 5 and Section 6

you can use following command lines:

sudo rm -r /etc/ldap/slapd.d
sudo mkdir /etc/ldap/slapd.d
sudo slaptest -f /etc/ldap/slapd.conf -F /etc/ldap/slapd.d
sudo chown -R openldap:openldap /etc/ldap/slapd.d
sudo service slapd restart

convert the static configuration to dynamic.

Before that, you should add serveral lines in slapd.conf to give access right of database config ie. cn=config
as,
########
database config
access to *
    by dn.exact="gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth" manage
    by * none

before line "database monitor"

The access directive means you can use "-Y EXTERNAL" with sudo to do ldap_add and ldapmodify command.
Otherwise, you should set exactly distinguish names to acess database cn=config, or you will get
insufficient permission(50) errors.

When you finish these, after restart the slapd service, your openldap is based on configuration from slapd.d

if you get any error from service restart process, most of failure comes from file owner or permission settings.

Enjoy!