The server response was: 503 This mail server requires authentication when attempting to send to a non-local e-mail address

I am not sure why now I am getting this error all of a sudden.  Our ISP had changed some of their mail server around, and I think this should still work but it's not:

    Public Function SendMail(ByVal userid As String) As Boolean
        Dim obUser As New BusinessLayer.BUser
        obUser.GetUser(SqlTypes.SqlGuid.Parse(userid))
        Dim obMail As New Web.Mail.MailMessage
        obMail.To = obUser.Email
        obMail.From = "postmaster@sss.net "
        obMail.Subject = "New Registration"
        obMail.BodyFormat = MailFormat.Html
        obMail.Body = "<b>Note : This mail is autogenerated. So do not reply.</b>" & _
            "<br>Dear " & obUser.FirstName & " " & obUser.MiddleName & " " & obUser.LastName & ",<br><br>" & "Thanks for registering with us." & _
            "<br><br>Your user name is <b>" & obUser.Email & "</b><br><br> Password is <b>" & obUser.Password & "</b><br><br>" & _
            "Please visit <a href='http://www.elbalazo.net'>sss.net</a> to login and " & _
            "access your account.<br><br>"
        SmtpMail.SmtpServer = "mail." & System.Configuration.ConfigurationSettings.AppSettings("DomainNameDisplay")
        SmtpMail.Send(obMail)
        obMail = Nothing
    End Function

ERROR: The server rejected one or more recipient addresses. The server response was: 503 This mail server requires authentication when attempting to send to a non-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server.