use google smtp as mailer

the only reason why I'm writing this is because I keep forgetting the link

https://myaccount.google.com/apppasswords

you need to set 2FA in your google account before you can set the apppasswords.

for development purpose, I will appsettings.json file to store the credential like suck

  "EmailSettings": {
    "MailServer": "smtp.gmail.com",
    "MailPort": 587,
    "SenderName": "aspnet-test",
    "FromEmail": "youreffingaddress@gmail.com",
    "Password": "xxxx xxxx xxxx xxxx"
  },

create private method and apply it your way. in this case I'm using microsoft Identity so the private method SendEmailAsync() will send email confirmation to newly registered user

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
#nullable disable

using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.Identity;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
using Microsoft.AspNetCore.WebUtilities;
using Microsoft.EntityFrameworkCore;
using System.ComponentModel.DataAnnotations;
using System.Net;
using System.Net.Mail;
using System.Text;
using System.Text.Encodings.Web;

namespace mambang.Areas.Identity.Pages.Account
{
    public class RegisterModel : PageModel
    {
        private readonly SignInManager<IdentityUser> _signInManager;
        private readonly UserManager<IdentityUser> _userManager;
        private readonly IUserStore<IdentityUser> _userStore;
        private readonly IUserEmailStore<IdentityUser> _emailStore;
        private readonly ILogger<RegisterModel> _logger;
        private readonly IConfiguration _configuration;

        public RegisterModel(
            UserManager<IdentityUser> userManager,
            IUserStore<IdentityUser> userStore,
            SignInManager<IdentityUser> signInManager,
            ILogger<RegisterModel> logger,
            IConfiguration configuration)
        {
            _userManager = userManager;
            _userStore = userStore;
            _emailStore = GetEmailStore();
            _signInManager = signInManager;
            _logger = logger;
            _configuration = configuration;
        }

        /// <summary>
        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
        ///     directly from your code. This API may change or be removed in future releases.
        /// </summary>
        [BindProperty]
        public InputModel Input { get; set; }

        /// <summary>
        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
        ///     directly from your code. This API may change or be removed in future releases.
        /// </summary>
        public string ReturnUrl { get; set; }

        /// <summary>
        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
        ///     directly from your code. This API may change or be removed in future releases.
        /// </summary>
        public IList<AuthenticationScheme> ExternalLogins { get; set; }

        /// <summary>
        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
        ///     directly from your code. This API may change or be removed in future releases.
        /// </summary>
        public class InputModel
        {
            /// <summary>
            ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
            ///     directly from your code. This API may change or be removed in future releases.
            /// </summary>
            [Required]
            [EmailAddress]
            [Display(Name = "Email")]
            public string Email { get; set; }

            [Required]
            [DataType(DataType.Text)]
            [RegularExpression(@"^(?!.*(?i)Deleted User).*$", ErrorMessage = "'Deleted User' is not allowed.")]
            public string UserName { get; set; }

            /// <summary>
            ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
            ///     directly from your code. This API may change or be removed in future releases.
            /// </summary>
            [Required]
            [StringLength(100, ErrorMessage = "The {0} must be at least {2} and at max {1} characters long.", MinimumLength = 6)]
            [DataType(DataType.Password)]
            [Display(Name = "Password")]
            public string Password { get; set; }

            /// <summary>
            ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
            ///     directly from your code. This API may change or be removed in future releases.
            /// </summary>
            [DataType(DataType.Password)]
            [Display(Name = "Confirm password")]
            [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
            public string ConfirmPassword { get; set; }
        }


        public async Task OnGetAsync(string returnUrl = null)
        {
            ReturnUrl = returnUrl;
            ExternalLogins = (await _signInManager.GetExternalAuthenticationSchemesAsync()).ToList();
        }

        public async Task<IActionResult> OnPostAsync(string returnUrl = null)
        {
            returnUrl ??= Url.Content("~/");
            ExternalLogins = (await _signInManager.GetExternalAuthenticationSchemesAsync()).ToList();
            if (ModelState.IsValid)
            {

                var existingUsers = await _userManager.Users.Where(u => u.Email == Input.Email).ToListAsync();
                var existingUserName = await _userManager.FindByNameAsync(Input.UserName);

                if (existingUsers.Count > 1)
                {
                    ModelState.AddModelError(string.Empty, "Multiple users with this email address exist.");
                    return Page();
                }

                if (existingUsers.Count == 1 || existingUserName != null)
                {
                    ModelState.AddModelError(string.Empty, "User already exists.");
                    return Page();
                }

                var user = CreateUser();

                await _userStore.SetUserNameAsync(user, Input.UserName, CancellationToken.None);
                await _emailStore.SetEmailAsync(user, Input.Email, CancellationToken.None);
                var result = await _userManager.CreateAsync(user, Input.Password);

                if (result.Succeeded)
                {
                    var addRolesResult = await _userManager.AddToRoleAsync(user, "User");
                    _logger.LogInformation("User created a new account with password.");

                    var userId = await _userManager.GetUserIdAsync(user);
                    var code = await _userManager.GenerateEmailConfirmationTokenAsync(user);
                    code = WebEncoders.Base64UrlEncode(Encoding.UTF8.GetBytes(code));
                    var callbackUrl = Url.Page(
                        "/Account/ConfirmEmail",
                        pageHandler: null,
                        values: new { area = "Identity", userId = userId, code = code, returnUrl = returnUrl },
                        protocol: Request.Scheme);

                    await SendEmailAsync(Input.Email, "Confirm your email",
                    //$"Please confirm your account by <a href='{HtmlEncoder.Default.Encode(callbackUrl)}'>clicking here</a>.");
                    $"Please confirm your account by <a href='{HtmlEncoder.Default.Encode(callbackUrl)}'>clicking here</a>.", true);

                    if (_userManager.Options.SignIn.RequireConfirmedAccount)
                    {
                        return RedirectToPage("RegisterConfirmation", new { email = Input.Email, returnUrl = returnUrl });
                    }
                    else
                    {
                        await _signInManager.SignInAsync(user, isPersistent: false);
                        return LocalRedirect(returnUrl);
                    }
                }
                foreach (var error in result.Errors)
                {
                    ModelState.AddModelError(string.Empty, error.Description);
                }
            }

            // If we got this far, something failed, redisplay form
            return Page();
        }

        private Task SendEmailAsync(string ToEmail, string Subject, string Body, bool IsBodyHtml = false)
        {
            try
            {
                string MailServer = _configuration["EmailSettings:MailServer"];
                string FromEmail = _configuration["EmailSettings:FromEmail"];
                string Password = _configuration["EmailSettings:Password"];
                int Port = int.Parse(_configuration["EmailSettings:MailPort"]);
                var client = new SmtpClient(MailServer, Port)
                {
                    Credentials = new NetworkCredential(FromEmail, Password),
                    EnableSsl = true,
                };
                MailMessage mailMessage = new MailMessage(FromEmail, ToEmail, Subject, Body)
                {
                    IsBodyHtml = IsBodyHtml
                };
                return client.SendMailAsync(mailMessage);
            }
            catch (Exception)
            {
                Console.WriteLine("error sampah!");
                throw;
            }
        }


        private IdentityUser CreateUser()
        {
            try
            {
                return Activator.CreateInstance<IdentityUser>();
            }
            catch
            {
                throw new InvalidOperationException($"Can't create an instance of '{nameof(IdentityUser)}'. " +
                    $"Ensure that '{nameof(IdentityUser)}' is not an abstract class and has a parameterless constructor, or alternatively " +
                    $"override the register page in /Areas/Identity/Pages/Account/Register.cshtml");
            }
        }

        private IUserEmailStore<IdentityUser> GetEmailStore()
        {
            if (!_userManager.SupportsUserEmail)
            {
                throw new NotSupportedException("The default UI requires a user store with email support.");
            }
            return (IUserEmailStore<IdentityUser>)_userStore;
        }
    }
}

happy coding!