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diff --git a/lib/pleroma/web/activity_pub/mrf/anti_mention_spam_policy.ex b/lib/pleroma/web/activity_pub/mrf/anti_mention_spam_policy.ex
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+++ b/lib/pleroma/web/activity_pub/mrf/anti_mention_spam_policy.ex
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+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.MRF.AntiMentionSpamPolicy do
+  alias Pleroma.Config
+  alias Pleroma.User
+  require Pleroma.Constants
+
+  @behaviour Pleroma.Web.ActivityPub.MRF.Policy
+
+  defp user_has_posted?(%User{} = u), do: u.note_count > 0
+
+  defp user_has_age?(%User{} = u) do
+    user_age_limit = Config.get([:mrf_antimentionspam, :user_age_limit], 30_000)
+    diff = NaiveDateTime.utc_now() |> NaiveDateTime.diff(u.inserted_at, :millisecond)
+    diff >= user_age_limit
+  end
+
+  defp good_reputation?(%User{} = u) do
+    user_has_age?(u) and user_has_posted?(u)
+  end
+
+  # copied from HellthreadPolicy
+  defp get_recipient_count(message) do
+    recipients = (message["to"] || []) ++ (message["cc"] || [])
+
+    follower_collection =
+      User.get_cached_by_ap_id(message["actor"] || message["attributedTo"]).follower_address
+
+    if Enum.member?(recipients, Pleroma.Constants.as_public()) do
+      recipients =
+        recipients
+        |> List.delete(Pleroma.Constants.as_public())
+        |> List.delete(follower_collection)
+
+      {:public, length(recipients)}
+    else
+      recipients =
+        recipients
+        |> List.delete(follower_collection)
+
+      {:not_public, length(recipients)}
+    end
+  end
+
+  defp object_has_recipients?(%{"object" => object} = activity) do
+    {_, object_count} = get_recipient_count(object)
+    {_, activity_count} = get_recipient_count(activity)
+    object_count + activity_count > 0
+  end
+
+  defp object_has_recipients?(object) do
+    {_, count} = get_recipient_count(object)
+    count > 0
+  end
+
+  @impl true
+  def filter(%{"type" => "Create", "actor" => actor} = activity) do
+    with {:ok, %User{local: false} = u} <- User.get_or_fetch_by_ap_id(actor),
+         {:has_mentions, true} <- {:has_mentions, object_has_recipients?(activity)},
+         {:good_reputation, true} <- {:good_reputation, good_reputation?(u)} do
+      {:ok, activity}
+    else
+      {:ok, %User{local: true}} ->
+        {:ok, activity}
+
+      {:has_mentions, false} ->
+        {:ok, activity}
+
+      {:good_reputation, false} ->
+        {:reject, "[AntiMentionSpamPolicy] User rejected"}
+
+      {:error, _} ->
+        {:reject, "[AntiMentionSpamPolicy] Failed to get or fetch user by ap_id"}
+
+      e ->
+        {:reject, "[AntiMentionSpamPolicy] Unhandled error #{inspect(e)}"}
+    end
+  end
+
+  # in all other cases, pass through
+  def filter(message), do: {:ok, message}
+
+  @impl true
+  def describe, do: {:ok, %{}}
+end