• 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

    “Write the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw

    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