function PosCh(ch: Char; const S: String): Integer;

var

  I: Integer;

begin

  for I:=1 to Length(S) do

    if S[I] = ch then

    begin

      result := I;

      Exit;

    end;

  result := 0;

end;

function ExtractOwner(const S: String): String;

var

  P: Integer;

begin

  result := '';

  P := PosCh('@', S);

  if P > 0 then

    result := Copy(S, 1, P - 1);

end;

procedure TForm1.FormCreate(Sender: TObject);

begin

showmessage(ExtractOwner('abc123@163.com'));

end;