Email is sent in Asp.net but an error is generated

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ASP.NET

Hi all ,

I am sending a registration confirmation email to the user after he registers.The email is being sent through gmail smtp.But an error is generated after that and it is :
The SMTP server requires a secure connection or the client was not authenticated.
This is the code :

Try
           Dim user As MembershipUser = Membership.GetUser(CreateUserWizard1.UserName.ToString())
           Dim userId As Guid = DirectCast(user.ProviderUserKey, Guid)
           Dim smtp As New SmtpClient()

           smtp.Host = "smtp.gmail.com"
           smtp.Port = 587
           smtp.EnableSsl = True
           smtp.DeliveryMethod = SmtpDeliveryMethod.Network
           smtp.UseDefaultCredentials = False
           smtp.Credentials = New NetworkCredential("ahmad.bastas@gmail.com", "password")
           ' Set MailMessage fields.
           e.Message.IsBodyHtml = True
           e.Message.Subject = "Allo Magazine : Registration Confirmation Email"
           ' Replace placeholder text in message body with information
           ' provided by the user.
           e.Message.Body = e.Message.Body.Replace("<%AC%>", userId.ToString())
           smtp.Send(e.Message)
           'after all of this happens we take the querystring when user presses on activation link and then see if he is not approved we approve him

       Catch ex As Exception
           Response.Write(ex.Message)

       End Try

And this is what in the create user wizard :

<MailDefinition Priority="High" BodyFileName="~/ConfirmationEmailHTML/ConfirmationEmail.htm"
                       Subject="Allo Magazine : Registration Confirmation Email" From="ahmad.bastas@gmail.com">
                   </MailDefinition>

So what is the error ?

Thanks in advance.