For LDAP backends, iRedMail uses its own LDAP schema file/structure by default, so if your 389 directory server works fine with this LDAP schema file, iRedMail and iRedAdmin-Pro should works fine too.
The default LDAP schema file is shipped in iRedMail. e.g. iRedMail-0.8.4/samples/iredmail.schema. Or you can check it out online:
https://bitbucket.org/zhb/iredmail/src/ … at=default