An email provider: one module that is a mishka_installer plugin on the "mail:providers" event and implements the callbacks below. MishkaCmsCore.Mail never names a provider; a site picks one on the settings page (mailer.provider) and every email of that site goes through it.

defmodule MyExtension.Mail.Postmark do
  use MishkaCmsCore.Mail.Provider, id: "postmark", label: "Postmark"

  @impl true
  def settings do
    [secret("mailer.postmark.server_token", gettext_noop("Postmark server token"))]
  end

  @impl true
  def ready?(site_id), do: not is_nil(Settings.secret("mailer.postmark.server_token", site_id))

  @impl true
  def deliver(email, site_id, _opts) do
    token = Settings.secret("mailer.postmark.server_token", site_id)
    MishkaCmsCore.Mailer.deliver(email, adapter: Swoosh.Adapters.Postmark, api_key: token)
  end
end

Supervise the module in the extension's application, like any other plugin. use adds the plugin half — the "mail:providers" registration, see MishkaCmsCore.ProviderRegistry — plus secret/3, which builds a provider setting the way the core's are built.

Settings

settings/0 declares the provider's own fields, shown on the settings page's Email tab under its label. Each scope sets its own (resolve: :own): a site never sends with the installation's key. A key is a :secret setting; read it with MishkaCmsCore.Settings.secret/2, never get/3.

Delivering

deliver/3 sends one Swoosh.Email for site_id and returns {:ok, message_id} (nil when the provider gives none) or {:error, reason}. opts carries :idempotency_key and :purpose. retry?/1 says whether a failure may succeed on a later attempt (a rate limit, a timeout) or will fail the same way again (a bad key, an unverified domain).

The registered providers

all/0, offered/0 and fetch/1 ask the "mail:providers" event which providers are installed: the core's own — Resend and, in development, the DevMailbox — register the same way an extension's would, so stopping a provider's plugin in the installer takes it off the settings page and out of delivery. Nothing is cached: the event is asked when the settings page loads and when an email is sent.