diff --git a/app/mailers/application_mailer.rb b/app/mailers/application_mailer.rb
index 3504f1a7a..0eb908467 100644
--- a/app/mailers/application_mailer.rb
+++ b/app/mailers/application_mailer.rb
@@ -3,6 +3,10 @@ class ApplicationMailer < ActionMailer::Base
   default from: "demarches-simplifiees.fr <#{CONTACT_EMAIL}>"
   layout 'mailer'
 
+  rescue_from Net::SMTPSyntaxError do |_error|
+    message.perform_deliveries = false
+  end
+
   # Attach the procedure logo to the email (if any).
   # Returns the attachment url.
   def attach_logo(procedure)
diff --git a/spec/mailers/application_mailer_spec.rb b/spec/mailers/application_mailer_spec.rb
new file mode 100644
index 000000000..9cfa295f9
--- /dev/null
+++ b/spec/mailers/application_mailer_spec.rb
@@ -0,0 +1,20 @@
+RSpec.describe ApplicationMailer, type: :mailer do
+  describe 'dealing with invalid emails' do
+    let(:dossier) { create(:dossier, procedure: build(:simple_procedure)) }
+    subject { DossierMailer.notify_new_draft(dossier) }
+
+    describe 'invalid emails are not sent' do
+      before do
+        allow_any_instance_of(DossierMailer)
+          .to receive(:notify_new_draft)
+          .and_raise(Net::SMTPSyntaxError)
+      end
+
+      it { expect(subject.message).to be_an_instance_of(ActionMailer::Base::NullMail) }
+    end
+
+    describe 'valid emails are sent' do
+      it { expect(subject.message).not_to be_an_instance_of(ActionMailer::Base::NullMail) }
+    end
+  end
+end