inc/class-Iac_Threaded_Mails.php
public function message_reference_headers( $headers, $iac_options, $item_ID ) {

		$type = ( 'iac_comment_headers' == current_filter() )
			? 'comment'
			: 'post';
		$parent = NULL;

		switch ( $type ) {
			case 'post' :
				$item = get_post( $item_ID );
				if ( 0 != $item->post_parent )
					$parent = get_post( $post_parent );
				break;
			case 'comment' :
				$item = get_comment( $item_ID );
				if ( 0 != $item->comment_parent ) {
					$parent = get_comment( $item->comment_parent );
				} else {
					$parent = get_post( $item->comment_post_ID );
					$type   = 'post';
				}
				break;
		}

		if ( ! $parent )
			return $headers;

		$headers[ 'References' ]  = '<' . Iac_Mail_ID::generate_ID( $type, $parent ) . '>';
		$headers[ 'In-Reply-To' ] = '<' . Iac_Mail_ID::generate_ID( $type, $parent ) . '>';

		return $headers;
	}
public function message_reference_headers( $headers, $iac_options, $item_ID ) {

		$type = ( 'iac_comment_headers' == current_filter() )
			? 'comment'
			: 'post';
		$parent = NULL;

		switch ( $type ) {
			case 'post' :
				$item = get_post( $item_ID );
				if ( 0 != $item->post_parent )
					$parent = get_post( $post_parent );
				break;
			case 'comment' :
				$item = get_comment( $item_ID );
				if ( 0 != $item->comment_parent ) {
					$parent = get_comment( $item->comment_parent );
				} else {
					$parent = get_post( $item->comment_post_ID );
					$type   = 'post';
				}
				break;
		}

		if ( ! $parent )
			return $headers;

		$headers[ 'References' ]  = '<' . Iac_Mail_ID::generate_ID( $type, $parent ) . '>';
		$headers[ 'In-Reply-To' ] = '<' . Iac_Mail_ID::generate_ID( $type, $parent ) . '>';

		return $headers;
	}
apply reference headers

@param array $headers
@param array $iac_options
@param int $item_ID
@return array