SQL Server 2019

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CREATE TABLE customer
             (email varchar(256));
INSERT INTO customer
            (email)
            VALUES ('alice@alice.com'),
                   ('bob@bob.org'),
                   ('charlie@charlie.net');
3 rows affected

SELECT *
       FROM customer;


email


alice@alice.com

bob@bob.org

charlie@charlie.net


UPDATE c
       SET c.email = replace(c.email, concat('.', tld.tld), '.mydomain.com')
       FROM customer c
            INNER JOIN (VALUES ('com'),
                               ('org'),
                               ('net')) tld (tld)
                       ON c.email LIKE concat('%.', tld.tld)
       WHERE c.email NOT LIKE '%.mydomain.com';
3 rows affected

SELECT *
       FROM customer;


email


alice@alice.mydomain.com

bob@bob.mydomain.com

charlie@charlie.mydomain.com