<person>
    <email-address>abc@test.com</email-address>
</person>

If XML.Tables("person").Select("email-address='" + Data Value + "'").Length = 1 Then
      Throw New Exception("Please enter another email address")
End If

XML.Tables("person").Select("email & Chr(150) &address....

If XML.Tables("person").Select("[email-address]='" + Data Value + "'").Length = 1 Then
    Throw New Exception("Please enter another email address")
End If