The optimization process and recommendations:

1. Avoid Subqueries (query line: 8): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, it's recommended to join a newly created temporary table that holds the data, which also includes the relevant search index.
2. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
3. Explicitly ORDER BY After GROUP BY (modified query below): By default, the database sorts all 'GROUP BY col1, col2, ...' queries as if you specified 'ORDER BY col1, col2, ...' in the query as well. If a query includes a GROUP BY clause but you want to avoid the overhead of sorting the result, you can suppress sorting by specifying 'ORDER BY NULL'.
4. Prefer IN Clause Over OR Conditions (modified query below): Using an IN clause is far more efficient than OR conditions, when comparing a column to more than one optional values. When using an IN clause, the database sorts the list of values and uses a quick binary search.

Optimal indexes for this query:

ALTER TABLE `correspondence_package_details` ADD INDEX `correspondence_det_idx_correspondence_details_key` (`correspondence_id`,`details_key`);
ALTER TABLE `correspondence_package_header` ADD INDEX `correspondence_hea_idx_create_date` (`create_date`);
ALTER TABLE `correspondence_package_header` ADD INDEX `correspondence_hea_idx_manual_id_account_id` (`manual_package_id`,`account_id`);

The optimized query:

SELECT
        GROUP_CONCAT(`correspondence_package_details`.`details_value`),
        `correspondence_package_details`.details_key 
    FROM
        `correspondence_package_details` 
    WHERE
        `correspondence_package_details`.`correspondence_id` IN (
            SELECT
                GROUP_CONCAT(`correspondence_package_header`.`correspondence_id`) 
            FROM
                `correspondence_package_header` 
            WHERE
                `correspondence_package_header`.create_date BETWEEN '2017-11-01' AND '2017-11-30' 
            GROUP BY
                `correspondence_package_header`.`manual_package_id`,
                `correspondence_package_header`.account_id 
            ORDER BY
                NULL
        ) 
        AND (
            `correspondence_package_details`.`details_key` IN (
                'to', 'cc'
            )
        ) 
    GROUP BY
        `correspondence_package_details`.details_key 
    ORDER BY
        NULL