diff --git a/accounts/forms.py b/accounts/forms.py
index c5efdcd..d11c888 100644
--- a/accounts/forms.py
+++ b/accounts/forms.py
@@ -9,9 +9,23 @@ class RegistrationForm(UserCreationForm):
         label=_("Email address"),
         widget=forms.TextInput(),
         required=True,
-        help_text=_("Please enter a valid email address ending with `@crans.org` or `@ens-paris-saclay.fr`."),
+        help_text=_(
+            "Please enter a valid email address ending with `@crans.org` or "
+            "`@ens-paris-saclay.fr`."
+        ),
     )
 
+    def clean_email(self):
+        """
+        Check that the email address ends with a trusted domain.
+        """
+        email = self.cleaned_data.get("email")
+        if not email.endswith("@crans.org") and not email.endswith("@ens-paris-saclay.fr"):
+            raise forms.ValidationError(
+                _("Must end with `@crans.org` or `@ens-paris-saclay.fr`.")
+            )
+        return email
+
     class Meta:
         model = User
         fields = ["username", "password1", "password2", "email"]