e-mail add überprüfung

Private Sub CommandButton1_Click()
Dim i As Integer
With Worksheets("tabelle1")
i = .UsedRange.Rows.Count + 1
.Cells(i, 1).Value = TextBox1.Value
End With
End Sub

Private Sub CommandButton2_Click()
UserForm3.Hide
End Sub

Private Sub CommandButton3_Click()
Dim a As String
i = a
If InStr(a, "@") > 1 Then      ? hier müsst das problem liegen ?
MsgBox "Email gültig!"
Else
MsgBox "Email ungültig!"
End If
End Sub

Private Sub TextBox1_Change()
End Sub