Visual Basic Smtp Error

hi guys, basically im making a program that sends an email to the user which has the score of the test he has done, it worked previously but now it is starting to say this error for some reason which is "The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. " and i cant understand why. * note: this didnt happen before

Dim SMTP As New SmtpClient("smtp.gmail.com") ' this declares the smtp server name
        Dim SmtpServer As New SmtpClient()
        Dim mail As New MailMessage()
        mail.Subject = "Biol Aid Answer Results"
        mail.To.Add(TextBox1.Text)
        mail.From = New MailAddress(TextBox2.Text)
        mail.Body = "Student Name: " & "signinform.textbox1" & vbNewLine & "Section:" & "Populations" & vbNewLine & "Score:" & " quesnum "                                                    ' this is what the message will be.
        SmtpServer.EnableSsl = True



        SMTP.EnableSsl = True
        SMTP.Credentials = New System.Net.NetworkCredential(TextBox2.Text, TextBox3.Text) 'username and password
        SMTP.Port = "587"                                                           'this is the google mail port number
        SMTP.Send(mail)                                                             'this sends the message