You need to add an LDAP address book in main.inc.php to access Active Directory. Something like this:
$rcmail_config['ldap_public']['Domain Addresses'] = array(
'name' => 'Domain Addresses',
'hosts' => array(mydc.domain.com'),
'port' => 389,
'use_tls' => false,
'user_specific' => true,
'base_dn' => 'OU=Accounts,DC=domain,DC=com',
'bind_dn' => 'ldap@domain.com',
'bind_pass' => 'ldappassword',
'writable' => false,
// If writable is true then these fields need to be populated:
// LDAP_Object_Classes, required_fields, LDAP_rdn
'LDAP_Object_Classes' => array("top", "inetOrgPerson"), // To create a new contact these are the object classes to specify (or any other classes you wish to use).
'required_fields' => array("cn", "sn", "mail"), // The required fields needed to build a new contact as required by the object classes (can include additional fields not required by
the object classes).
'LDAP_rdn' => 'mail', // The RDN field that is used for new entries, this field needs to be one of the search_fields, the base of base_dn is appended to the RDN to insert into the LDAP
directory.
'ldap_version' => 3, // using LDAPv3
'search_fields' => array('mail', 'cn'), // fields to search in
'name_field' => 'cn', // this field represents the contact's name
'email_field' => 'mail', // this field represents the contact's e-mail
'surname_field' => 'sn', // this field represents the contact's last name
'firstname_field' => 'givenName', // this field represents the contact's first name
'sort' => 'cn', // The field to sort the listing by.
'scope' => 'sub', // search mode: sub|base|list
'filter' => '(&(objectClass=user)(!(objectClass=computer)))', // used for basic listing (if not empty) and will be &'d with search queries. example: status=act
'fuzzy_search' => true); // server allows wildcard search
