Samba 4 has its own LDAP server, this would allow for an almost direct replacement of OpenLDAP.
The major benefits would be that iredmail ldap would be fully openchange compatible, ie. this would provide native outlook compatibility (mapi support).
Currently Zentyal is the only mail solution to support this as standard.
An alternative could be to write a conversion/sync script between openldap and samba4, however this seems pointless.