The app's code that receives a message: one module for each kind (support, replies, photographs), named in the routes (Letterbox.Routing).

defmodule MyApp.SitesMailbox do
  use Letterbox.Mailbox

  @impl true
  def process(message) do
    with %Site{} = site <- Sites.by_inbound_token(Letterbox.Address.tag(message.recipient)),
         %User{} = user <- Sites.member_by_email(site, message.verified_from) do
      Enum.each(message.attachments, &Media.add_from_email(site, user, &1))
    else
      nil -> {:bounce, :unknown_sender}
    end
  end
end

process/1 runs out of the request, in a job (Letterbox.Runner), and gives:

• :ok - done: the message is "delivered";
• {:bounce, reason} - refused, for good: the message is "bounced", and not tried again;
• {:error, reason}, or an exception - it is tried again, up to :max_attempts times, then "failed".

Anything else is :ok. A mailbox is tried again with the same message, so what it does should be safe to do twice (an attachment kept twice is one file, with Storehouse's :existing).