I would use VBA to 'validate' an email address.
Assuming that A1 will be the cell containing the email address, copy this event handler to the clipboard:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$A$1" And Target.Value <> "" Then
If Len(Target.Value) - Len(Application.Substitute(Target, ".", "")) =0 Or _
Len(Target) - Len(Application.Substitute(Target, "@", "")) <> 1 Then
MsgBox "Invalid Email Address", vbCritical, "Rekey Email Address"
Target.Value = ""
Select the worksheet to contain the validation and right click the sheet tab at the bottom.
Select 'View Code'.
Paste the event handler into the white editing area to the right.
Close the VBE (red button top right).
Enter an email address in A1. If it does not contain at least one '.', or contains more than one '@', an error message will be displayed and the entry deleted from A1.