emails: validate that email addresses are deliverable.

Reject email addresses that known to be undeliverable during signup.
Some users signup with invalid email addresses, which causes the welcome
email (which contains the email confirmation link) to bounce. Too many
bounces hurt our ability to send mail.

We check that an email address is undeliverable by checking if the
domain has a mail server and if the server returns an invalid address
error when attempting to send mail. This isn't foolproof since some
servers don't return an error if the address doesn't exist. If the
checks fail we know the address is bad, but if the checks pass that
doesn't guarantee the address is good. However, this is still good
enough to filter out bad addresses for popular providers like Gmail and
Microsoft that do return nonexistent address errors.

The address existence check requires being able to connect to mail
servers over port 25. This may fail if your network blocks port 25,
which many home ISPs and hosting providers do by default.

app/controllers/users_controller.rb

@@ -59,22 +59,31 @@ class UsersController < ApplicationController
   end
 
   def create
-    @user = authorize User.new(last_ip_addr: CurrentUser.ip_addr, **permitted_attributes(User))
+    @user = authorize User.new(
+      last_ip_addr: CurrentUser.ip_addr,
+      name: params[:user][:name],
+      password: params[:user][:password],
+      password_confirmation: params[:user][:password_confirmation]
+    )
 
-    if !Danbooru.config.enable_recaptcha? || verify_recaptcha(model: @user)
-      @user.save
-      if @user.errors.empty?
-        session[:user_id] = @user.id
-        UserMailer.welcome_user(@user).deliver_later
-      else
-        flash[:notice] = "Sign up failed: #{@user.errors.full_messages.join("; ")}"
-      end
-      set_current_user
-      respond_with(@user)
-    else
-      flash[:notice] = "Sign up failed"
-      redirect_to new_user_path
+    if params[:user][:email].present?
+      @user.email_address = EmailAddress.new(address: params[:user][:email])
     end
+
+    if Danbooru.config.enable_recaptcha? && !verify_recaptcha(model: @user)
+      flash[:notice] = "Sign up failed"
+    elsif @user.email_address&.invalid?(:deliverable)
+      flash[:notice] = "Sign up failed: email address is invalid or doesn't exist"
+      @user.errors[:base] << @user.email_address.errors.full_messages.join("; ")
+    elsif !@user.save
+      flash[:notice] = "Sign up failed: #{@user.errors.full_messages.join("; ")}"
+    else
+      session[:user_id] = @user.id
+      UserMailer.welcome_user(@user).deliver_later
+      set_current_user
+    end
+
+    respond_with(@user)
   end
 
   def update