I have a email database

The @sp_lock solution will work, unless there is another occurence in the '.com' in the eamil addres (eg. in user name priror @). So I suggest to find the position in reversed email addres as below.

WITH SampleData AS (
 SELECT 'test1@gmail.comwww' AS email UNION ALL
 SELECT 'test2.com@gmail.comzzz' 
)
SELECT
 left(email, len(email) - charindex('moc.', reverse(email)) + 1)
FROM SampleData

Update after @Usman comment: This will ignore any not .com domains. However the ideal way would be to use a CLR Regex with single niice expression. :-)

WITH SampleData AS (
 SELECT 'test1@gmail.comwww' AS email UNION ALL
 SELECT 'test2.com@gmail.comzzz' UNION ALL
 SELECT 'test3.com@gmail.cozzz'
)
SELECT
 LEFT(email, len(email) - charindex('moc.', reverse(substring(email, patindex('%@%.com%', email), sign( patindex('%@%.com%', email)) * len(email)))) + 1)
FROM SampleData

Hey guys you missed the most important part that the OP tagged it in mysql not MSSQL ;)

So just translating Pavel's solution with an additional check

SELECT  --THE FOLLOWING EXPRESSION CAN BE PART OF WHERE CLAUSE WHILE UPDATING THE DATA
        CASE WHEN LEFT(REVERSE(email), LOCATE('.', REVERSE(email))) LIKE '%moc.'
             THEN LEFT(email,
                        CHAR_LENGTH(email) - LOCATE('moc.', REVERSE(email)) + 1)
             ELSE email
        END
FROM    SampleData

I am still not sure it is fool proof solution and any mysql guru could come up with a better solution since REGEXP is native to mysql.(@Pavel I am sure you are going to like REGEXP being native in mysql. So would you like to take a shot at this :))

Is this what you are after?

Is it basic and would need changing for other addresses like .co.uk .net etc

DECLARE @string varchar(50) 
SET @string = 'jl@test.comjy'
SELECT 
 LEFT( @string,charindex('.com',@string) + 3)