Why my Email sending through C#(.NET) are receiving in spam instead of inbox

    public static void Send_email(this string email)
    {
        MailMessage mail = new MailMessage();
        mail.To.Add(email);
        mail.From = new MailAddress("Sender_email@gmail.com");
        mail.Subject = "Welcome to eStudiez!!!";
        mail.Body = "<h3>You have registered <span 
        style=\"color:green;\">Successfully</span><br />Please wait for the admin to approve you </h3>";
        mail.IsBodyHtml = true;
       
        SmtpClient smtp = new SmtpClient();
        smtp.Port = 587; // 25 465
        smtp.EnableSsl = true;
        smtp.UseDefaultCredentials = false;
        smtp.Host = "smtp.gmail.com";
        smtp.Credentials = new System.Net.NetworkCredential("Sender_email@gmail.com", "apppassword");
        smtp.Send(mail);
        smtp.Dispose();
     }

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Email is sending through C#(.NET) but on the other end it is receiving in spam section