diff --git a/app/controllers/users/profil_controller.rb b/app/controllers/users/profil_controller.rb
index f976a8775..13c9c7f20 100644
--- a/app/controllers/users/profil_controller.rb
+++ b/app/controllers/users/profil_controller.rb
@@ -12,8 +12,9 @@ module Users
     def update_email
       if @current_user.update(update_email_params)
         flash.notice = t('devise.registrations.update_needs_confirmation')
-      # to avoid leaking who has signed in
       elsif @current_user.errors&.details&.dig(:email)&.any? { |e| e[:error] == :taken }
+        UserMailer.account_already_taken(@current_user, requested_email).deliver_later
+        # avoid leaking information about whether an account with this email exists or not
         flash.notice = t('devise.registrations.update_needs_confirmation')
       else
         flash.alert = @current_user.errors.full_messages
@@ -27,5 +28,9 @@ module Users
     def update_email_params
       params.require(:user).permit(:email)
     end
+
+    def requested_email
+      update_email_params[:email]
+    end
   end
 end
diff --git a/app/mailers/user_mailer.rb b/app/mailers/user_mailer.rb
index 2304d41fd..d0f7b91f6 100644
--- a/app/mailers/user_mailer.rb
+++ b/app/mailers/user_mailer.rb
@@ -8,4 +8,12 @@ class UserMailer < ApplicationMailer
 
     mail(to: user.email, subject: @subject)
   end
+
+  def account_already_taken(user, requested_email)
+    @user = user
+    @requested_email = requested_email
+    @subject = "Changement d’adresse email"
+
+    mail(to: requested_email, subject: @subject)
+  end
 end
diff --git a/app/views/user_mailer/account_already_taken.haml b/app/views/user_mailer/account_already_taken.haml
new file mode 100644
index 000000000..85cbf69ac
--- /dev/null
+++ b/app/views/user_mailer/account_already_taken.haml
@@ -0,0 +1,20 @@
+- content_for(:title, @subject)
+
+%p
+  Bonjour,
+
+%p
+  L’utilisateur « #{@user.email} » a demandé le changement de son adresse vers « #{@requested_email} ».
+
+%p
+  Malheureusement, votre compte « #{@requested_email} » existe déjà. Nous ne pouvons pas fusionner automatiquement vos comptes.
+
+%p
+  %strong Nous ne pouvons donc pas effectuer le changement d’adresse email.
+
+%p
+  Si vous n'êtes pas à l’origine de cette demande, vous pouvez ignorer ce message. Et si vous avez besoin d’assistance, n’hésitez pas à nous contacter à
+  = succeed '.' do
+    = mail_to CONTACT_EMAIL
+
+= render partial: "layouts/mailers/signature"
diff --git a/spec/controllers/users/profil_controller_spec.rb b/spec/controllers/users/profil_controller_spec.rb
index e151e2f82..38f46f2dd 100644
--- a/spec/controllers/users/profil_controller_spec.rb
+++ b/spec/controllers/users/profil_controller_spec.rb
@@ -1,6 +1,8 @@
 require 'spec_helper'
 
 describe Users::ProfilController, type: :controller do
+  include ActiveJob::TestHelper
+
   let(:user) { create(:user) }
 
   before { sign_in(user) }
@@ -34,13 +36,17 @@ describe Users::ProfilController, type: :controller do
     end
 
     context 'when the mail is already taken' do
-      let!(:user2) { create(:user) }
+      let(:existing_user) { create(:user) }
 
       before do
-        patch :update_email, params: { user: { email: user2.email } }
+        perform_enqueued_jobs do
+          patch :update_email, params: { user: { email: existing_user.email } }
+        end
         user.reload
       end
 
+      it { expect(user.unconfirmed_email).to be_nil }
+      it { expect(ActionMailer::Base.deliveries.last.to).to eq([existing_user.email]) }
       it { expect(response).to redirect_to(profil_path) }
       it { expect(flash.notice).to eq(I18n.t('devise.registrations.update_needs_confirmation')) }
     end
diff --git a/spec/mailers/previews/user_mailer_preview.rb b/spec/mailers/previews/user_mailer_preview.rb
index 4204d63be..e99994fa4 100644
--- a/spec/mailers/previews/user_mailer_preview.rb
+++ b/spec/mailers/previews/user_mailer_preview.rb
@@ -3,6 +3,10 @@ class UserMailerPreview < ActionMailer::Preview
     UserMailer.new_account_warning(user)
   end
 
+  def account_already_taken
+    UserMailer.account_already_taken(user, 'dircab@territoires.gouv.fr')
+  end
+
   private
 
   def user
diff --git a/spec/mailers/user_mailer_spec.rb b/spec/mailers/user_mailer_spec.rb
new file mode 100644
index 000000000..dd48194a5
--- /dev/null
+++ b/spec/mailers/user_mailer_spec.rb
@@ -0,0 +1,21 @@
+require "rails_helper"
+
+RSpec.describe UserMailer, type: :mailer do
+  let(:user) { build(:user) }
+
+  describe '.new_account_warning' do
+    subject { described_class.new_account_warning(user) }
+
+    it { expect(subject.to).to eq([user.email]) }
+    it { expect(subject.body).to include(user.email) }
+  end
+
+  describe '.account_already_taken' do
+    let(:requested_email) { 'new@exemple.fr' }
+
+    subject { described_class.account_already_taken(user, requested_email) }
+
+    it { expect(subject.to).to eq([requested_email]) }
+    it { expect(subject.body).to include(requested_email) }
+  end
+end