Fix message id format

lib/philomena/users/user_notifier.ex

@@ -3,21 +3,25 @@ defmodule Philomena.Users.UserNotifier do
   alias Philomena.Mailer
 
   defp deliver(to, subject, body) do
-    id =
-      :crypto.strong_rand_bytes(16)
-      |> Base.encode16()
-      |> String.downcase()
-
     Email.new(
       to: to,
       from: {"noreply", mailer_address()},
       subject: subject,
       text_body: body
     )
-    |> Email.header("Message-ID", id)
+    |> Email.header("Message-ID", message_id())
     |> Mailer.deliver_later()
   end
 
+  defp message_id do
+    id =
+      :crypto.strong_rand_bytes(16)
+      |> Base.encode16()
+      |> String.downcase()
+
+    "#{id}.#{mailer_address()}"
+  end
+
   defp mailer_address do
     Application.get_env(:philomena, :mailer_address)
   end