fix: Handle email sending failures gracefully

Adds exception handling for SMTPRecipientsRefused to prevent
unhandled exceptions when email delivery fails. Provides
user feedback on email verification failure and removes
registration record if the initial verification email fails.

Improves user experience by allowing system to continue even
if an email address is invalid or blocked.

src/synapse_registration/registration/signals.py

@@ -7,6 +7,8 @@ from .models import UserRegistration
 
 import requests
 
+from smtplib import SMTPRecipientsRefused
+
 
 @receiver(post_save, sender=UserRegistration)
 def handle_status_change(sender, instance, created, **kwargs):
@@ -35,21 +37,17 @@ def handle_status_change(sender, instance, created, **kwargs):
                     headers={"Authorization": f"Bearer {settings.SYNAPSE_ADMIN_TOKEN}"},
                 )
 
+            try:
                 send_mail(
                     "Registration Approved",
                     f"Congratulations, {instance.username}! Your registration at {settings.MATRIX_DOMAIN} has been approved.",
                     settings.DEFAULT_FROM_EMAIL,
                     [instance.email],
                 )
+            except SMTPRecipientsRefused:
+                pass
 
         elif status == UserRegistration.STATUS_DENIED:
-            send_mail(
-                "Registration Denied",
-                f"Sorry, your registration request at {settings.MATRIX_DOMAIN} has been denied.",
-                settings.DEFAULT_FROM_EMAIL,
-                [instance.email],
-            )
-
             response = requests.put(
                 f"{settings.SYNAPSE_SERVER}/_synapse/admin/v2/users/@{instance.username}:{settings.MATRIX_DOMAIN}",
                 json={"deactivated": True},
@@ -63,3 +61,13 @@ def handle_status_change(sender, instance, created, **kwargs):
                     settings.DEFAULT_FROM_EMAIL,
                     [settings.ADMIN_EMAIL],
                 )
+
+            try:
+                send_mail(
+                    "Registration Denied",
+                    f"Sorry, your registration request at {settings.MATRIX_DOMAIN} has been denied.",
+                    settings.DEFAULT_FROM_EMAIL,
+                    [instance.email],
+                )
+            except SMTPRecipientsRefused:
+                pass

src/synapse_registration/registration/views.py

@@ -11,6 +11,7 @@ import requests
 
 from secrets import token_urlsafe
 from datetime import datetime, timedelta
+from smtplib import SMTPRecipientsRefused
 
 
 class LandingPageView(TemplateView):
@@ -76,15 +77,18 @@ class EmailInputView(FormView):
 
         token = token_urlsafe(32)
 
-        UserRegistration.objects.create(
+        registration = UserRegistration.objects.create(
             username=self.request.session["username"],
             email=email,
             token=token,
             ip_address=ip_address,
         )
+
         verification_link = self.request.build_absolute_uri(
             reverse_lazy("verify_email", args=[token])
         )
+
+        try:
             send_mail(
                 "Verify your email",
                 f"Click the link to verify your email: {verification_link}",
@@ -93,6 +97,11 @@ class EmailInputView(FormView):
             )
             return render(self.request, "registration/email_sent.html")
 
+        except SMTPRecipientsRefused:
+            form.add_error("email", "Your email address is invalid, not accepting mail, or blocked by our mail server.")
+            registration.delete()
+            return self.form_invalid(form)
+
 
 class VerifyEmailView(View):
     def get(self, request, token):