Вы можете передать значение в функцию, подобную этой:
Function blnValidEmail(strText As String) As Boolean
Dim intPosAt As Integer
Dim intPosDot As Integer
'finds the position of the @ symbol'
intPosAt = InStr(strText, "@")
'finds the position of the last full stop'
'checks the last full stop because you might'
'have an address like jane.doe@something.com'
intPosDot = InStrRev(strText, ".")
'makes sure that both exist'
If intPosAt > 0 And intPosDot > 0 Then
'makes sure that there is a fullstop after the @'
If intPosDot > intPosAt Then
blnValidEmail= True
End If
End If
End Function