I improved iRedAdmin-Pro-1.3.0.
May i know your existing LDAP structure used for Drupal/etc? e.g.
ldap suffix: dc=example,dc=com
ou contains users: ou=Users,domainName=yourdomain.com,dc=example,dc=com
The rule is, let ldap queries query 'ou=Users', and set search scope to subtree. And it should work as expected.
I didn't test iRedMail yet, it's too late for me here. You should test it yourself. Sorry
Also, may i know how you manage your accounts used for iRedMail/Drupal/etc? And, How drupal perform LDAP query? Is it forced to use 'cn' as RDN? if not, i think just change Drupal is easier.