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

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

		$item = ( 'post' == $type )
			? get_post( $item_ID )
			: get_comment( $item_ID );

		$headers[ 'Message-ID' ] =  '<' . Iac_Mail_ID::generate_ID( $type, $item ) . '>';

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

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

		$item = ( 'post' == $type )
			? get_post( $item_ID )
			: get_comment( $item_ID );

		$headers[ 'Message-ID' ] =  '<' . Iac_Mail_ID::generate_ID( $type, $item ) . '>';

		return $headers;
	}
apply the ID header

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