diff --git a/app/assets/javascripts/discourse/app/components/composer-messages.js b/app/assets/javascripts/discourse/app/components/composer-messages.js
index 45aa08ed95f..40832b5df17 100644
--- a/app/assets/javascripts/discourse/app/components/composer-messages.js
+++ b/app/assets/javascripts/discourse/app/components/composer-messages.js
@@ -5,8 +5,10 @@ import LinkLookup from "discourse/lib/link-lookup";
 import { not } from "@ember/object/computed";
 import { scheduleOnce } from "@ember/runloop";
 import showModal from "discourse/lib/show-modal";
+import { ajax } from "discourse/lib/ajax";
 
 let _messagesCache = {};
+let _recipient_names = [];
 
 export default Component.extend({
   classNameBindings: [":composer-popup-container", "hidden"],
@@ -18,6 +20,7 @@ export default Component.extend({
   _similarTopicsMessage: null,
   _yourselfConfirm: null,
   similarTopics: null,
+  usersNotSeen: null,
 
   hidden: not("composer.viewOpenOrFullscreen"),
 
@@ -119,6 +122,53 @@ export default Component.extend({
     const composer = this.composer;
     if (composer.get("privateMessage")) {
       const recipients = composer.targetRecipientsArray;
+      const recipient_names = recipients
+        .filter((r) => r.type === "user")
+        .map(({ name }) => name);
+
+      if (
+        recipient_names.length > 0 &&
+        recipient_names.length !== _recipient_names.length &&
+        !recipient_names.every((v, i) => v === _recipient_names[i])
+      ) {
+        _recipient_names = recipient_names;
+
+        ajax(`/composer_messages/user_not_seen_in_a_while`, {
+          type: "GET",
+          data: {
+            usernames: recipient_names,
+          },
+        }).then((response) => {
+          if (
+            response.user_count > 0 &&
+            this.get("usersNotSeen") !== response.usernames.join("-")
+          ) {
+            this.set("usersNotSeen", response.usernames.join("-"));
+            this.messagesByTemplate["education"] = undefined;
+
+            let usernames = [];
+            response.usernames.forEach((username, index) => {
+              usernames[
+                index
+              ] = `<a class='mention' href='/u/${username}'>@${username}</a>`;
+            });
+
+            let body_key = "composer.user_not_seen_in_a_while.single";
+            if (response.user_count > 1) {
+              body_key = "composer.user_not_seen_in_a_while.multiple";
+            }
+            const message = composer.store.createRecord("composer-message", {
+              id: "user-not-seen",
+              templateName: "education",
+              body: I18n.t(body_key, {
+                usernames: usernames.join(", "),
+                time_ago: response.time_ago,
+              }),
+            });
+            this.send("popup", message);
+          }
+        });
+      }
 
       if (
         recipients.length > 0 &&
diff --git a/app/assets/javascripts/discourse/tests/acceptance/composer-messages-test.js b/app/assets/javascripts/discourse/tests/acceptance/composer-messages-test.js
new file mode 100644
index 00000000000..25f9cae229e
--- /dev/null
+++ b/app/assets/javascripts/discourse/tests/acceptance/composer-messages-test.js
@@ -0,0 +1,41 @@
+import {
+  acceptance,
+  exists,
+  query,
+} from "discourse/tests/helpers/qunit-helpers";
+import { click, triggerKeyEvent, visit } from "@ember/test-helpers";
+import { test } from "qunit";
+import I18n from "I18n";
+
+acceptance("Composer - Messages", function (needs) {
+  needs.user();
+  needs.pretender((server, helper) => {
+    server.get("/composer_messages/user_not_seen_in_a_while", () => {
+      return helper.response({
+        user_count: 1,
+        usernames: ["charlie"],
+        time_ago: "1 year ago",
+      });
+    });
+  });
+
+  test("Shows warning in composer if user hasn't been seen in a long time.", async function (assert) {
+    await visit("/u/charlie");
+    await click("button.compose-pm");
+    assert.ok(
+      !exists(".composer-popup"),
+      "composer warning is not shown by default"
+    );
+    await triggerKeyEvent(".d-editor-input", "keyup", "Space");
+    assert.ok(exists(".composer-popup"), "shows composer warning message");
+    assert.ok(
+      query(".composer-popup").innerHTML.includes(
+        I18n.t("composer.user_not_seen_in_a_while.single", {
+          usernames: ['<a class="mention" href="/u/charlie">@charlie</a>'],
+          time_ago: "1 year ago",
+        })
+      ),
+      "warning message has correct body"
+    );
+  });
+});
diff --git a/app/controllers/composer_messages_controller.rb b/app/controllers/composer_messages_controller.rb
index c5d4de3c647..b8303014559 100644
--- a/app/controllers/composer_messages_controller.rb
+++ b/app/controllers/composer_messages_controller.rb
@@ -17,4 +17,22 @@ class ComposerMessagesController < ApplicationController
 
     render_json_dump(json, rest_serializer: true)
   end
+
+  def user_not_seen_in_a_while
+    usernames = params.require(:usernames)
+    users = ComposerMessagesFinder.user_not_seen_in_a_while(usernames)
+    user_count = users.count
+    warning_message = nil
+
+    if user_count > 0
+      message_locale = if user_count == 1
+        "education.user_not_seen_in_a_while.single"
+      else
+        "education.user_not_seen_in_a_while.multiple"
+      end
+    end
+
+    json = { user_count: user_count, usernames: users, time_ago: FreedomPatches::Rails4.time_ago_in_words(SiteSetting.pm_warn_user_last_seen_months_ago.month.ago, true, scope: :'datetime.distance_in_words_verbose') }
+    render_json_dump(json)
+  end
 end
diff --git a/config/locales/client.en.yml b/config/locales/client.en.yml
index 5789279a7d2..9159b3cbc78 100644
--- a/config/locales/client.en.yml
+++ b/config/locales/client.en.yml
@@ -2275,6 +2275,9 @@ en:
         body: "Right now this message is only being sent to yourself!"
       slow_mode:
         error: "This topic is in slow mode. You already posted recently; you can post again in %{timeLeft}."
+      user_not_seen_in_a_while:
+        single: "The person you are messaging, <b>%{usernames}</b>, hasn’t been seen here in a very long time – %{time_ago}. They may not receive your message. You may wish to seek out alternate methods of contacting %{usernames}."
+        multiple: "The following people you are messaging: <b>%{usernames}</b>, haven’t been seen here in a very long time – %{time_ago}. They may not receive your message. You may wish to seek out alternate methods of contacting them."
 
       admin_options_title: "Optional staff settings for this topic"
 
diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index ea49d6f106a..5558bc40d26 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -2149,6 +2149,8 @@ en:
 
     disable_avatar_education_message: "Disable education message for changing avatar."
 
+    pm_warn_user_last_seen_months_ago: "When creating a new PM warn users when target recepient has not been seen more than n months ago."
+
     suppress_uncategorized_badge: "Don't show the badge for uncategorized topics in topic lists."
 
     header_dropdown_category_count: "How many categories can be displayed in the header dropdown menu."
diff --git a/config/routes.rb b/config/routes.rb
index f5eccd9af5e..6b9e4958f28 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -387,6 +387,7 @@ Discourse::Application.routes.draw do
     end
     get "session/scopes" => "session#scopes"
     get "composer_messages" => "composer_messages#index"
+    get "composer_messages/user_not_seen_in_a_while" => "composer_messages#user_not_seen_in_a_while"
 
     resources :static
     post "login" => "static#enter"
diff --git a/config/site_settings.yml b/config/site_settings.yml
index 93fb4a5f415..f2106b26382 100644
--- a/config/site_settings.yml
+++ b/config/site_settings.yml
@@ -2272,6 +2272,7 @@ uncategorized:
   get_a_room_threshold: 3
   dominating_topic_minimum_percent: 20
   disable_avatar_education_message: false
+  pm_warn_user_last_seen_months_ago: 24
 
   global_notice:
     default: ""
diff --git a/lib/composer_messages_finder.rb b/lib/composer_messages_finder.rb
index 06ce89634e0..30acc8c7cda 100644
--- a/lib/composer_messages_finder.rb
+++ b/lib/composer_messages_finder.rb
@@ -212,6 +212,10 @@ class ComposerMessagesFinder
     }
   end
 
+  def self.user_not_seen_in_a_while(usernames)
+    User.where(username_lower: usernames).where("last_seen_at < ?", SiteSetting.pm_warn_user_last_seen_months_ago.months.ago).pluck(:username).sort
+  end
+
   private
 
   def educate_reply?(type)
diff --git a/spec/lib/composer_messages_finder_spec.rb b/spec/lib/composer_messages_finder_spec.rb
index e6bbac2144c..7ff75f30f34 100644
--- a/spec/lib/composer_messages_finder_spec.rb
+++ b/spec/lib/composer_messages_finder_spec.rb
@@ -501,4 +501,26 @@ RSpec.describe ComposerMessagesFinder do
       expect(edit_post_finder.find).to eq(nil)
     end
   end
+
+  describe '#user_not_seen_in_a_while' do
+    fab!(:user_1) { Fabricate(:user, last_seen_at: 3.years.ago) }
+    fab!(:user_2) { Fabricate(:user, last_seen_at: 2.years.ago) }
+    fab!(:user_3) { Fabricate(:user, last_seen_at: 6.months.ago) }
+
+    before do
+      SiteSetting.pm_warn_user_last_seen_months_ago = 24
+    end
+
+    it 'returns users that have not been seen recently' do
+      users = ComposerMessagesFinder.user_not_seen_in_a_while([user_1.username, user_2.username, user_3.username])
+      expect(users).to contain_exactly(user_1.username, user_2.username)
+    end
+
+    it 'accounts for pm_warn_user_last_seen_months_ago site setting' do
+      SiteSetting.pm_warn_user_last_seen_months_ago = 30
+      users = ComposerMessagesFinder.user_not_seen_in_a_while([user_1.username, user_2.username, user_3.username])
+      expect(users).to contain_exactly(user_1.username)
+    end
+  end
+
 end
diff --git a/spec/requests/composer_messages_controller_spec.rb b/spec/requests/composer_messages_controller_spec.rb
index 3ce4fe613da..22af26f4563 100644
--- a/spec/requests/composer_messages_controller_spec.rb
+++ b/spec/requests/composer_messages_controller_spec.rb
@@ -30,4 +30,45 @@ RSpec.describe ComposerMessagesController do
       end
     end
   end
+
+  describe '#user_not_seen_in_a_while' do
+    fab!(:user_1) { Fabricate(:user, last_seen_at: 3.years.ago) }
+    fab!(:user_2) { Fabricate(:user, last_seen_at: 2.years.ago) }
+    fab!(:user_3) { Fabricate(:user, last_seen_at: 6.months.ago) }
+
+    it 'requires you to be logged in' do
+      get '/composer_messages/user_not_seen_in_a_while.json', params: { usernames: [user_1.username, user_2.username, user_3.username] }
+      expect(response.status).to eq(403)
+    end
+
+    context 'when logged in' do
+      let!(:user) { sign_in(Fabricate(:user)) }
+
+      before do
+        SiteSetting.pm_warn_user_last_seen_months_ago = 24
+      end
+
+      it 'requires usernames parameter to be present' do
+        get '/composer_messages/user_not_seen_in_a_while.json'
+        expect(response.status).to eq(400)
+      end
+
+      it 'returns users that have not been seen recently' do
+        get '/composer_messages/user_not_seen_in_a_while.json', params: { usernames: [user_1.username, user_2.username, user_3.username] }
+        expect(response.status).to eq(200)
+        json = response.parsed_body
+        expect(json["user_count"]).to eq(2)
+        expect(json["usernames"]).to contain_exactly(user_1.username, user_2.username)
+      end
+
+      it 'accounts for pm_warn_user_last_seen_months_ago site setting' do
+        SiteSetting.pm_warn_user_last_seen_months_ago = 30
+        get '/composer_messages/user_not_seen_in_a_while.json', params: { usernames: [user_1.username, user_2.username, user_3.username] }
+        expect(response.status).to eq(200)
+        json = response.parsed_body
+        expect(json["user_count"]).to eq(1)
+        expect(json["usernames"]).to contain_exactly(user_1.username)
+      end
+    end
+  end
 end