This works quite nicely too:
SELECT
EmployeeID,
Email,
DomainLHS = SUBSTRING(Email, x.PosStart,x.PosEnd-x.PosStart),
DomainRHS = SUBSTRING(Email, y.PosStart,y.PosEnd-y.PosStart)
FROM @t
CROSS APPLY (
SELECT PosStart = 1+CHARINDEX('@',EMAIL,1),
PosEnd = NULLIF(CHARINDEX('.',EMAIL,CHARINDEX('@',EMAIL,1)),0)
) x
CROSS APPLY (
SELECT PosStart = NULLIF(1+CHARINDEX('@',EMAIL,x.PosEnd),1),
PosEnd = NULLIF(CHARINDEX('.',EMAIL,CHARINDEX('@',EMAIL,x.PosEnd)),0)
) y
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden