It's really helpful if you provide the DDL/DML, in an easy to reproduce format.

DECLARE @Emails TABLE (Email NVARCHAR(100), EmailID INT);
INSERT INTO @Emails (Email, EmailID) VALUES
('[email protected]', 1234),('[email protected]', 1235), ('[email protected]', 1236), ('[email protected]', 6780);

SELECT e.Email, e.EmailID, CASE WHEN PATINDEX('%[A-Z|a-z|0-9]@%', e.Email) > 0 THEN 1 ELSE 0 END AS AlpaNumericBeforeAt, LEFT(e.Email,CHARINDEX('@',e.Email)-1) AS UserName,
       SUBSTRING(e.Email,CHARINDEX('@',e.Email)+1,LEN(e.Email)-(CHARINDEX('@',e.Email)+CHARINDEX('.',REVERSE(e.Email)))) AS Domain, 
       REVERSE(LEFT(REVERSE(e.Email),CHARINDEX('.',REVERSE(e.Email))-1)) AS Extenstion
  FROM @Emails e;

Here we're using the string manipulation functions LEFT , SUBSTRING , PATINDEX , REVERSE and CHARINDEX to slice and dice the emails up. https://learn.microsoft.com/en-us/sql/t-sql/functions/string-functions-transact-sql?view=sql-server-ver16 .

I've assumed A-Z a-z and 0-9 covers the alpha-numeric range you wanted.

This is probably a job for your favorite ETL tool, but could work in a pinch. You may want to consider storing the results rather than regenerating them all the time.