Это не использует регулярное выражение:
Option Explicit
Function isEmail(ByVal data As String) As Boolean
Dim u As Long, temp As String, brr
isEmail = False
temp = Replace(data, ".", ",")
If InStr(temp, ",") = 0 Then Exit Function
brr = Split(temp, ",")
u = UBound(brr)
If u = 1 Or u = 2 Then
isEmail = True
Exit Function
End If
End Function