Check and fail domains with null MX records

mock.patch("dns.resolver.LRUCache.get") breaks the DNS check in a way that didn't fail the deliverability check before, but it does now, so the mock is replaced with something else.

Author: JoshData

README.md

@@ -100,7 +100,7 @@ addresses are allowed when passing a `bytes`) and:
 
 When an email address is not valid, `validate_email` raises either an
 `EmailSyntaxError` if the form of the address is invalid or an
-`EmailUndeliverableError` if the domain name does not resolve. Both
+`EmailUndeliverableError` if the domain name fails the DNS check. Both
 exception classes are subclasses of `EmailNotValidError`, which in turn
 is a subclass of `ValueError`.
 
@@ -113,14 +113,15 @@ one uses anymore even though they are still valid and deliverable, since
 they will probably give you grief if you're using email for login. (See
 later in the document about that.)
 
-The validator checks that the domain name in the email address resolves.
+The validator checks that the domain name in the email address has a
+(non-null) MX DNS record indicating that it is configured for email.
 There is nothing to be gained by trying to actually contact an SMTP
 server, so that's not done here. For privacy, security, and practicality
 reasons servers are good at not giving away whether an address is
 deliverable or not: email addresses that appear to accept mail at first
 can bounce mail after a delay, and bounced mail may indicate a temporary
 failure of a good email address (sometimes an intentional failure, like
-greylisting).
+greylisting). (A/AAAA-record fallback is also checked.)
 
 The function also accepts the following keyword arguments (default as
 shown):
@@ -129,7 +130,7 @@ shown):
     require the
     [SMTPUTF8](https://tools.ietf.org/html/rfc6531) extension.
 
-`check_deliverability=True`: Set to `False` to skip the domain name resolution check.
+`check_deliverability=True`: Set to `False` to skip the domain name MX DNS record check.
 
 `allow_empty_local=False`: Set to `True` to allow an empty local part (i.e.
     `@example.com`), e.g. for validating Postfix aliases.

email_validator/__init__.py

@@ -531,10 +531,19 @@ def dns_resolver_resolve_shim(domain, record):
             raise dns.exception.Timeout()
 
         try:
-            # Try resolving for MX records and get them in sorted priority order.
+            # Try resolving for MX records and get them in sorted priority order
+            # as (priority, qname) pairs.
             response = dns_resolver_resolve_shim(domain, "MX")
             mtas = sorted([(r.preference, str(r.exchange).rstrip('.')) for r in response])
             mx_fallback = None
+
+            # Do not permit delivery if there is only a "null MX" record (whose value is
+            # (0, ".") but we've stripped trailing dots, so the 'exchange' is just "").
+            mtas = [(preference, exchange) for preference, exchange in mtas
+                    if exchange != ""]
+            if len(mtas) == 0:
+                raise EmailUndeliverableError("The domain name %s does not accept email." % domain_i18n)
+
         except (dns.resolver.NoNameservers, dns.resolver.NXDOMAIN, dns.resolver.NoAnswer):
 
             # If there was no MX record, fall back to an A record.

tests/test_main.py

@@ -1,4 +1,3 @@
-from unittest import mock
 import dns.resolver
 import pytest
 from email_validator import EmailSyntaxError, EmailUndeliverableError, \
@@ -292,10 +291,6 @@ def test_dict_accessor():
     assert valid_email.as_dict()["original_email"] == input_email
 
 
-def test_deliverability_no_records():
-    assert validate_email_deliverability('example.com', 'example.com') == {'mx': [(0, '')], 'mx-fallback': None}
-
-
 def test_deliverability_found():
     response = validate_email_deliverability('gmail.com', 'gmail.com')
     assert response.keys() == {'mx', 'mx-fallback'}
@@ -307,10 +302,16 @@ def test_deliverability_found():
 
 
 def test_deliverability_fails():
+    # No MX record.
     domain = 'xkxufoekjvjfjeodlfmdfjcu.com'
     with pytest.raises(EmailUndeliverableError, match='The domain name {} does not exist'.format(domain)):
         validate_email_deliverability(domain, domain)
 
+    # Null MX record.
+    domain = 'example.com'
+    with pytest.raises(EmailUndeliverableError, match='The domain name {} does not accept email'.format(domain)):
+        validate_email_deliverability(domain, domain)
+
 
 def test_deliverability_dns_timeout():
     validate_email_deliverability.TEST_CHECK_TIMEOUT = True
@@ -379,25 +380,34 @@ def test_main_output_shim(monkeypatch, capsys):
     assert stdout == "b'An email address cannot have a period immediately after the @-sign.'\n"
 
 
-@mock.patch("dns.resolver.LRUCache.put")
-def test_validate_email__with_caching_resolver(mocked_put):
-    dns_resolver = caching_resolver()
-    validate_email("test@gmail.com", dns_resolver=dns_resolver)
-    assert mocked_put.called
+def test_validate_email__with_caching_resolver():
+    # unittest.mock.patch("dns.resolver.LRUCache.get") doesn't
+    # work --- it causes get to always return an empty list.
+    # So we'll mock our own way.
+    class MockedCache:
+        get_called = False
+        put_called = False
 
-    with mock.patch("dns.resolver.LRUCache.get") as mocked_get:
-        validate_email("test@gmail.com", dns_resolver=dns_resolver)
-        assert mocked_get.called
+        def get(self, key):
+            self.get_called = True
+            return None
 
+        def put(self, key, value):
+            self.put_called = True
 
-@mock.patch("dns.resolver.LRUCache.put")
-def test_validate_email__with_configured_resolver(mocked_put):
+    # Test with caching_resolver helper method.
+    mocked_cache = MockedCache()
+    dns_resolver = caching_resolver(cache=mocked_cache)
+    validate_email("test@gmail.com", dns_resolver=dns_resolver)
+    assert mocked_cache.put_called
+    validate_email("test@gmail.com", dns_resolver=dns_resolver)
+    assert mocked_cache.get_called
+
+    # Test with dns.resolver.Resolver instance.
     dns_resolver = dns.resolver.Resolver()
     dns_resolver.lifetime = 10
-    dns_resolver.cache = dns.resolver.LRUCache(max_size=1000)
+    dns_resolver.cache = MockedCache()
     validate_email("test@gmail.com", dns_resolver=dns_resolver)
-    assert mocked_put.called
-
-    with mock.patch("dns.resolver.LRUCache.get") as mocked_get:
-        validate_email("test@gmail.com", dns_resolver=dns_resolver)
-        assert mocked_get.called
+    assert mocked_cache.put_called
+    validate_email("test@gmail.com", dns_resolver=dns_resolver)
+    assert mocked_cache.get_called