I can't set up mail server authorization using openLdap, what should I do?

Question

System administration

good afternoon! there is a corporate postfix + dovecot + roundcube mail (I don’t know if there is sasl or not). All this business turns on centos 6.5. I didn't set it up. Next, there is an openldap server on centos 7. I set up Openldap myself according to the instructions . It has user entities uid=username,ou=group,dc=centos,dc=com. All entities have attributes: email, password, cn.
We set the task to set up authorization of mail accounts through openLdap. There is no experience with mail and openLdap, so everything is very unclear how to set it up. I roughly understand that it is necessary to specify in the postfix and dovecot settings to use the openLdap database. But someone sets up virtual users, someone uses sasl, thereby driving them into a complete stupor.
I ask you to help me understand the settings of this scheme: how it all should work, what and where to prescribe, because it will be necessary to transfer about 100 accounts along with their mail. How to correctly create entities in openldap for these purposes, what rights to set (will it be enough to read)?
I have been working with centos for about 3 months, so I don’t throw stones, I can be stupid because I don’t understand what’s what, from which side to approach this problem at all.