email address         | product code
----------------------+---------------
garry123@gmail.com    | A123A
garry123@gmail.com    | AB263
ada121@hotmail.com    | 45632A
ada121@hotmail.com    | 78YU
garry123@gmail.com    | 6543D

garry123@gmail.com | A123A,AB263,6543D
ada121@hotmail.com | 45632A,78YU

SELECT
    c.EmailAddress,
    od.concat(productcode) as Product_SKU
FROM
    OrderDetails od
JOIN
    Orders o ON od.OrderID = o.OrderID
JOIN
    Customers c ON c.CustomerID = o.CustomerID
WHERE
    o.OrderDate BETWEEN 01/01/2016 AND GETDATE()
GROUP BY
    c.EmailAddress

SELECT
    c.EmailAddress,
    productcode = STUFF((SELECT ',' + od.productcode
                         FROM Orderdetails od
                         WHERE c.EmailAddress = od.EmailAddress
                         FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'), 1, 1, '')
FROM
    OrderDetails od
JOIN
    Orders o ON od.OrderID = o.OrderID
JOIN
    Customers c ON c.CustomerID = o.CustomerID
WHERE
    o.OrderDate BETWEEN 01/01/2016 AND GETDATE()

declare @test table
(
email varchar(50),
address varchar(50)
)

insert into @test VALUES
('garry123@gmail.com','A123A'),
('garry123@gmail.com','AB263'),
('ada121@hotmail.com','45632A'),
('ada121@hotmail.com','78YU'),
('garry123@gmail.com','6543D')

SELECT DISTINCT
       email,
       Stuff((SELECT ', ' + address
              FROM   @test t2
              WHERE  t2.email  = t1.email
              FOR XML PATH('')), 1, 2, '') Address
FROM   @test t1