Ok, SendGrid, we'll do it your way.

.gitignore

@@ -35,3 +35,5 @@
 
 .env
 private/
+
+/config/master.key

app/mailboxes/application_mailbox.rb

@@ -0,0 +1,3 @@
+class ApplicationMailbox < ActionMailbox::Base
+  routing :all => :incomings
+end

app/mailboxes/incomings_mailbox.rb

@@ -0,0 +1,15 @@
+class IncomingsMailbox < ApplicationMailbox
+  def process
+    mail_attachments
+  end
+
+  def mail_body
+    mail.body.decoded
+  end
+
+  def mail_attachments
+    mail.mail_attachments.each do |attachment|
+      # your logic here
+    end
+  end
+end

config/credentials.yml.enc

@@ -0,0 +1 @@
+hlOhSyqh0KJoerA3ojBSyGK+uHvPMsisouAb9oRiX3GllB/R7F4gQjX0+Ipu/ixFRTMYdEl9biq+ZQt9y+4dUsfqzuMjfVkJp2aP8AjZO/3Rc7msUw67V3GLBLWFOdOgIk7to04Ta0OcJCDoqiNj/A1XHCLCdUCfe3E/UKp5XiStCocyvJexri/v4D3TmnvJVUjORdmIAI/8//vK9ElAxBvqLTWHKRWup2VUdqIHd+aelOtur2dHLJ6YO6LUMYSony5T4ubU2FUo1VnSeJU1jTiTPna8fegjhSUQPIrU2gQjdseljXiMhrlyL4y/42hLua/vVHydWACToj1rV3qYXZXg/Zh13rwyZSo2aip8nzJ9OTthihfx7aTsBTXPCxmPxImxSvHbYvBLqO5WIEIvLgf4yZUcFCBZ2cEZwgLn7ZnofRlkluzZxUkGqH0A--WeC7nRLgdXv4ar+D--HTpMvy6AYOiOGdTTfQNtTw==

config/environments/production.rb

@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 Rails.application.configure do
+  # Prepare the ingress controller used to receive mail
+  config.action_mailbox.ingress = :sendgrid
+
   # Settings specified here will take precedence over those in config/application.rb.
 
   # Code is not reloaded between requests.

db/migrate/20230726012825_create_active_storage_tables.active_storage.rb

@@ -0,0 +1,48 @@
+# This migration comes from active_storage (originally 20170806125915)
+class CreateActiveStorageTables < ActiveRecord::Migration[5.2]
+  def change
+    # Use Active Record's configured type for primary and foreign keys
+    primary_key_type, foreign_key_type = primary_and_foreign_key_types
+
+    create_table :active_storage_blobs, id: primary_key_type do |t|
+      t.string   :key,          null: false
+      t.string   :filename,     null: false
+      t.string   :content_type
+      t.text     :metadata
+      t.string   :service_name, null: false
+      t.bigint   :byte_size,    null: false
+      t.string   :checksum,     null: false
+      t.datetime :created_at,   null: false
+
+      t.index [ :key ], unique: true
+    end
+
+    create_table :active_storage_attachments, id: primary_key_type do |t|
+      t.string     :name,     null: false
+      t.references :record,   null: false, polymorphic: true, index: false, type: foreign_key_type
+      t.references :blob,     null: false, type: foreign_key_type
+
+      t.datetime :created_at, null: false
+
+      t.index [ :record_type, :record_id, :name, :blob_id ], name: "index_active_storage_attachments_uniqueness", unique: true
+      t.foreign_key :active_storage_blobs, column: :blob_id
+    end
+
+    create_table :active_storage_variant_records, id: primary_key_type do |t|
+      t.belongs_to :blob, null: false, index: false, type: foreign_key_type
+      t.string :variation_digest, null: false
+
+      t.index %i[ blob_id variation_digest ], name: "index_active_storage_variant_records_uniqueness", unique: true
+      t.foreign_key :active_storage_blobs, column: :blob_id
+    end
+  end
+
+  private
+    def primary_and_foreign_key_types
+      config = Rails.configuration.generators
+      setting = config.options[config.orm][:primary_key_type]
+      primary_key_type = setting || :primary_key
+      foreign_key_type = setting || :bigint
+      [primary_key_type, foreign_key_type]
+    end
+end

db/migrate/20230726012826_create_action_mailbox_tables.action_mailbox.rb

@@ -0,0 +1,14 @@
+# This migration comes from action_mailbox (originally 20180917164000)
+class CreateActionMailboxTables < ActiveRecord::Migration[6.0]
+  def change
+    create_table :action_mailbox_inbound_emails do |t|
+      t.integer :status, default: 0, null: false
+      t.string  :message_id, null: false
+      t.string  :message_checksum, null: false
+
+      t.timestamps
+
+      t.index [ :message_id, :message_checksum ], name: "index_action_mailbox_inbound_emails_uniqueness", unique: true
+    end
+  end
+end

db/schema.rb

@@ -2,19 +2,56 @@
 # of editing this file, please use the migrations feature of Active Record to
 # incrementally modify your database, and then regenerate this schema definition.
 #
-# This file is the source Rails uses to define your schema when running `rails
-# db:schema:load`. When creating a new database, `rails db:schema:load` tends to
+# This file is the source Rails uses to define your schema when running `bin/rails
+# db:schema:load`. When creating a new database, `bin/rails db:schema:load` tends to
 # be faster and is potentially less error prone than running all of your
 # migrations from scratch. Old migrations may fail to apply correctly if those
 # migrations use external dependencies or application code.
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2021_12_05_074007) do
+ActiveRecord::Schema.define(version: 2023_07_26_012826) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
 
+  create_table "action_mailbox_inbound_emails", force: :cascade do |t|
+    t.integer "status", default: 0, null: false
+    t.string "message_id", null: false
+    t.string "message_checksum", null: false
+    t.datetime "created_at", precision: 6, null: false
+    t.datetime "updated_at", precision: 6, null: false
+    t.index ["message_id", "message_checksum"], name: "index_action_mailbox_inbound_emails_uniqueness", unique: true
+  end
+
+  create_table "active_storage_attachments", force: :cascade do |t|
+    t.string "name", null: false
+    t.string "record_type", null: false
+    t.bigint "record_id", null: false
+    t.bigint "blob_id", null: false
+    t.datetime "created_at", null: false
+    t.index ["blob_id"], name: "index_active_storage_attachments_on_blob_id"
+    t.index ["record_type", "record_id", "name", "blob_id"], name: "index_active_storage_attachments_uniqueness", unique: true
+  end
+
+  create_table "active_storage_blobs", force: :cascade do |t|
+    t.string "key", null: false
+    t.string "filename", null: false
+    t.string "content_type"
+    t.text "metadata"
+    t.string "service_name", null: false
+    t.bigint "byte_size", null: false
+    t.string "checksum", null: false
+    t.datetime "created_at", null: false
+    t.index ["key"], name: "index_active_storage_blobs_on_key", unique: true
+  end
+
+  create_table "active_storage_variant_records", force: :cascade do |t|
+    t.bigint "blob_id", null: false
+    t.string "variation_digest", null: false
+    t.index ["blob_id", "variation_digest"], name: "index_active_storage_variant_records_uniqueness", unique: true
+  end
+
   create_table "awards", id: :serial, force: :cascade do |t|
     t.integer "badge_id"
     t.integer "user_id"
@@ -291,6 +328,8 @@
     t.index ["name"], name: "index_users_on_name", unique: true
   end
 
+  add_foreign_key "active_storage_attachments", "active_storage_blobs", column: "blob_id"
+  add_foreign_key "active_storage_variant_records", "active_storage_blobs", column: "blob_id"
   add_foreign_key "challenges", "users", column: "creator_id"
   add_foreign_key "discussions", "boards"
   add_foreign_key "discussions", "users"

spec/mailboxes/incomings_mailbox_spec.rb

@@ -0,0 +1,5 @@
+require 'rails_helper'
+
+RSpec.describe IncomingsMailbox, type: :mailbox do
+  pending "add some examples to (or delete) #{__FILE__}"
+end