Declare @test varchar(50)
Select @test = 'John Doe johndoe@email.com'
SELECT RIGHT(@test, CHARINDEX(' ', REVERSE(@test)-1))
or a safer approach (if there are strings without space separator):
Declare @test varchar(50)
Select @test = 'johndoe@email.com'
SELECT
CASE
WHEN CHARINDEX(' ', REVERSE(@test)) > 0 THEN RIGHT(@test, CHARINDEX(' ', REVERSE(@test))-1)
ELSE @test
END