Update cache handling

src/pretix/presale/forms/checkout.py

@@ -20,12 +20,16 @@ from pretix.base.validators import EmailBanlistValidator
 from pretix.presale.signals import contact_form_fields
 
 
-class EmailDNSValidator():
+class EmailDNSValidator:
+    msg = _('Please check your email domain, it does not look like "%(value)s" is able to receive emails.')
+
     def __call__(self, value):
         domain = value.split('@')[-1]
         works = cache.get(f"mail_domain_exists_{domain}")
-        if works:
+        if works == "true":
             return value
+        if works == "false":
+            raise ValidationError(self.msg, code='dns', params={'value': domain})
 
         resolver = dns.resolver.Resolver()
         resolver.lifetime = 0.5
@@ -36,13 +40,20 @@ class EmailDNSValidator():
                 if len(resolver.query(domain, record_type)):
                     cache.set(f"mail_domain_exists_{domain}", "true", 3600 * 24 * 7)
                     return value
-            except:
+            except (dns.resolver.NoAnswer, dns.resolver.NXDOMAIN, dns.resolver.YXDOMAIN):
+                # These errors represent a non-existing domain or entry, we want to continue with the
+                # other types of records and if all fail, we'll report an error.
                 continue
-        raise ValidationError(
-            _('Please check your email domain, it does not look like "%(value)s" is able to receive emails.'),
-            code='dns',
-            params={'value': domain},
-        )
+            except:
+                # Timeout, IO errors, etc.: We don't know what's happening! Let's not stop the user
+                # from buying and just not validate this. Let's cache this result for a short while
+                # to prevent issues during peak load
+                cache.set(f"mail_domain_exists_{domain}", "false", 60)
+                return value
+        # No valid record found, even though the requests did not fail. This domain will not receive email.
+        # Cache this result for a short time.
+        cache.set(f"mail_domain_exists_{domain}", "false", 300)
+        raise ValidationError(self.msg, code='dns', params={'value': domain})
 
 
 class ContactForm(forms.Form):