diff --git a/app/mailers/notification_mailer.rb b/app/mailers/notification_mailer.rb
index 89f79c1db..8f8bb6c57 100644
--- a/app/mailers/notification_mailer.rb
+++ b/app/mailers/notification_mailer.rb
@@ -7,6 +7,7 @@
 #
 class NotificationMailer < ApplicationMailer
   include ActionView::Helpers::SanitizeHelper
+  include ActionView::Helpers::TextHelper
 
   before_action :set_dossier
   before_action :set_services_publics_plus, only: :send_notification
@@ -67,7 +68,7 @@ class NotificationMailer < ApplicationMailer
         mail_template = @dossier.procedure.mail_template_for(params[:state])
 
         @email = @dossier.user_email_for(:notification)
-        @subject = mail_template.subject_for_dossier(@dossier)
+        @subject = truncate(mail_template.subject_for_dossier(@dossier), length: 100)
         @body = mail_template.body_for_dossier(@dossier)
         @actions = mail_template.actions_for_dossier(@dossier)
         @attachment = mail_template.attachment_for_dossier(@dossier)
diff --git a/app/models/concerns/mail_template_concern.rb b/app/models/concerns/mail_template_concern.rb
index e778c681d..a5035bbd8 100644
--- a/app/models/concerns/mail_template_concern.rb
+++ b/app/models/concerns/mail_template_concern.rb
@@ -10,7 +10,7 @@ module MailTemplateConcern
   end
 
   def subject_for_dossier(dossier)
-    replace_tags(subject, dossier)
+    replace_tags(subject, dossier).presence || replace_tags(self.class::DEFAULT_SUBJECT, dossier)
   end
 
   def body_for_dossier(dossier)
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 17d80a709..dc8be05ec 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -395,6 +395,19 @@ en:
         zone: This procedure is run by
       champs:
         value: Value
+      default_mail_attributes: &default_mail_attributes
+        hints:
+          subject: The generated subject will be truncated if it exceeds 100 characters.
+      mails/closed_mail:
+        << : *default_mail_attributes
+      mails/initiated_mail:
+        << : *default_mail_attributes
+      mails/received_mail:
+        << : *default_mail_attributes
+      mails/refused_mail:
+        << : *default_mail_attributes
+      mails/without_continuation_mail:
+        << : *default_mail_attributes
 
     errors:
       messages:
diff --git a/config/locales/fr.yml b/config/locales/fr.yml
index efd5e4d63..d4b864c28 100644
--- a/config/locales/fr.yml
+++ b/config/locales/fr.yml
@@ -392,6 +392,19 @@ fr:
         zone: La démarche est mise en œuvre par
       champs:
         value: Valeur du champ
+      default_mail_attributes: &default_mail_attributes
+        hints:
+          subject: "L’objet généré sera tronqué s’il dépasse 100 caractères."
+      mails/closed_mail:
+        << : *default_mail_attributes
+      mails/initiated_mail:
+        << : *default_mail_attributes
+      mails/received_mail:
+        << : *default_mail_attributes
+      mails/refused_mail:
+        << : *default_mail_attributes
+      mails/without_continuation_mail:
+        << : *default_mail_attributes
 
     errors:
       messages:
diff --git a/spec/mailers/notification_mailer_spec.rb b/spec/mailers/notification_mailer_spec.rb
index 005db343c..25e77eb4d 100644
--- a/spec/mailers/notification_mailer_spec.rb
+++ b/spec/mailers/notification_mailer_spec.rb
@@ -101,4 +101,27 @@ RSpec.describe NotificationMailer, type: :mailer do
       end
     end
   end
+
+  describe 'subject length' do
+    let(:procedure) { create(:simple_procedure, libelle: "My super long title " + ("xo " * 100)) }
+    let(:dossier) { create(:dossier, :en_instruction, :with_individual, :with_service, user: user, procedure: procedure) }
+    let(:email_template) { create(:closed_mail, subject:, body: 'Your dossier was accepted. Thanks.') }
+
+    before do
+      dossier.procedure.closed_mail = email_template
+    end
+
+    subject(:mail) { described_class.send_accepte_notification(dossier) }
+
+    context "subject is too long" do
+      let(:subject) { 'Un long libellé --libellé démarche--' }
+      it { expect(mail.subject.length).to be <= 100 }
+    end
+
+    context "subject should fallback to default" do
+      let(:subject) { "" }
+      it { expect(mail.subject).to match(/^Votre dossier .+ a été accepté \(My super long title/) }
+      it { expect(mail.subject.length).to be <= 100 }
+    end
+  end
 end