The optimization process and recommendations:

1. Avoid Selecting Unnecessary Columns (query line: 2): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.
2. Avoid Subqueries (query line: 4): 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.
3. Avoid Subqueries (query line: 17): 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.
4. 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.
5. 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'.

Optimal indexes for this query:

ALTER TABLE `chauntry` ADD INDEX `chauntry_idx_mailing_indicator` (`mailing_indicator`);
ALTER TABLE `chauntry` ADD INDEX `chauntry_idx_added` (`added`);
ALTER TABLE `chauntry` ADD INDEX `chauntry_idx_email` (`email`);

The optimized query:

SELECT
        * 
    FROM
        (SELECT
            DISTINCT chauntry.email,
            chauntry.title,
            chauntry.first_name,
            chauntry.last_name,
            'chauntry' AS source,
            chauntry.post_code AS postcode 
        FROM
            chauntry 
        WHERE
            chauntry.mailing_indicator = 1) AS x 
    JOIN
        (
            SELECT
                chauntry.email,
                Avg(chauntry.amount_paid) AS avg_paid,
                Count(*) AS no_times_booked,
                Count(DISTINCT (Date_format(chauntry.added,
                '%M %Y'))) AS unique_months 
            FROM
                chauntry 
            WHERE
                chauntry.added >= Now() - INTERVAL 1 year 
            GROUP BY
                chauntry.email 
            ORDER BY
                NULL
        ) AS y 
            ON x.email = y.email