Secure authentication error

[gmadeira]

I'm using the following code to send an email:

Dim SendMessage As New Net.Mail.SmtpClient
                Dim userToken As New Object
                Dim EmailMessage As New MailMessage
                With EmailMessage
                    .From = New MailAddress("[e-mail]")
                    .To.Add("[e-mail]")
                    .Subject = "Test"
                    .Body = "test"
                End With


                AddHandler SendMessage.SendCompleted, AddressOf sendMail_SendCompleted

                With SendMessage
                    .Host = "smtp.gmail.com"
                    .Port = 25
                    .UseDefaultCredentials = False
                    .Credentials = New Net.NetworkCredential("username", "password")
                    .EnableSsl = False
                End With

                'SendMessage.SendAsync(cbxFrom.Text, cbxTo.Text, tbSubject.Text, tbBody.Text, userToken)
                SendMessage.SendAsync(EmailMessage, userToken)
            End While

    Private Sub sendMail_SendCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.AsyncCompletedEventArgs)
        Dim userToken As Object = e.UserState 'This is how you know which send has completed, you pass some identifier in usertoken

        If e.Cancelled Then 'Operation was cancelled
            Debug.WriteLine("Email cancelled.")
        End If

        If e.Error IsNot Nothing Then 'An error occured
            Debug.WriteLine("Error: {0}", e.Error.ToString)
        Else 'No error occured
            Debug.WriteLine("Email sent!")
        End If
    End Sub

When I run this code I receive the following error:

"A first chance exception of type 'System.Net.Mail.SmtpException' occurred in System.dll
System.Net.Mail.SmtpException: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Must issue a STARTTLS command first. 5sm359599ybq.8
at System.Net.Mail.SendMailAsyncResult.End(IAsyncResult result)
at System.Net.Mail.SmtpClient.SendMailCallback(IAsyncResult result): Error: {0}
The thread 0x68c has exited with code 0 (0x0).
The thread 0x25a0 has exited with code 0 (0x0)."

What am I doing wrong ??? I already tested with defaultcredentials = true but does not work !