Use OpenLDAP as address book in Microsoft Outlook

Tested with Outlook 2007:

on account settings choose address book tab
click New
choose Internet directory service (LDAP)
Input the server name or IP address.
Check the box This server requires me to logon
Input the full LDAP dn of your mail account as login username. For example: mail=www@testserver.com,ou=Users,domainName=testserver.com,o=domains,dc=testserver,dc=com, dont forget to input the correct user password.
Click More Settings
Leave by default
Click Search
Choose Custom and input the LDAP dn or your mail domain: ou=Users,domainName=testserver.com,o=domains,dc=testserver,dc=com
Done.

Please make sure network port 389 (OpenLDAP service) is open to external network on your iRedMail server, it's blocked in iRedMail by default..