How to use variables inside a select (SQL Server)?
Derived table

SELECT
    EMail,
    hashbytes('SHA1', EMail) AS HashedEmail
FROM
    (
    SELECT LOWER(SUBSTRING([NAME], 4, 100)) + '@somedomain.com' as EMail
    FROM sometable
   ) foo


or CTE:

;WITH cEMail AS
(
    SELECT LOWER(SUBSTRING([NAME], 4, 100)) + '@somedomain.com' as EMail
    FROM sometable
)
SELECT
    EMail,
    hashbytes('SHA1', EMail) AS HashedEmail
FROM
    cEMail


However, in this case I'd consider using a computed column with the hash precalculated