diff --git a/synapse/rulecheck/__init__.py b/synapse/rulecheck/__init__.py
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/synapse/rulecheck/domain_rule_checker.py b/synapse/rulecheck/domain_rule_checker.py
new file mode 100644
index 0000000000..3caa6b34cb
--- /dev/null
+++ b/synapse/rulecheck/domain_rule_checker.py
@@ -0,0 +1,100 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import logging
+
+from synapse.config._base import ConfigError
+
+logger = logging.getLogger(__name__)
+
+
+class DomainRuleChecker(object):
+    """
+    A re-implementation of the SpamChecker that prevents users in one domain from
+    inviting users in other domains to rooms, based on a configuration.
+
+    Takes a config in the format:
+
+    spam_checker:
+        module: "rulecheck.DomainRuleChecker"
+        config:
+          domain_mapping:
+            "inviter_domain": [ "invitee_domain_permitted", "other_domain_permitted" ]
+            "other_inviter_domain": [ "invitee_domain_permitted" ]
+          default: False
+        }
+
+    Don't forget to consider if you can invite users from your own domain.
+    """
+
+    def __init__(self, config):
+        self.domain_mapping = config["domain_mapping"] or {}
+        self.default = config["default"]
+
+    def check_event_for_spam(self, event):
+        """Implements synapse.events.SpamChecker.check_event_for_spam
+        """
+        return False
+
+    def user_may_invite(self, inviter_userid, invitee_userid, room_id):
+        """Implements synapse.events.SpamChecker.user_may_invite
+        """
+        inviter_domain = self._get_domain_from_id(inviter_userid)
+        invitee_domain = self._get_domain_from_id(invitee_userid)
+
+        if inviter_domain not in self.domain_mapping:
+            return self.default
+
+        return invitee_domain in self.domain_mapping[inviter_domain]
+
+    def user_may_create_room(self, userid):
+        """Implements synapse.events.SpamChecker.user_may_create_room
+        """
+        return True
+
+    def user_may_create_room_alias(self, userid, room_alias):
+        """Implements synapse.events.SpamChecker.user_may_create_room_alias
+        """
+        return True
+
+    def user_may_publish_room(self, userid, room_id):
+        """Implements synapse.events.SpamChecker.user_may_publish_room
+        """
+        return True
+
+    @staticmethod
+    def parse_config(config):
+        """Implements synapse.events.SpamChecker.parse_config
+        """
+        if "default" in config:
+            return config
+        else:
+            raise ConfigError("No default set for spam_config DomainRuleChecker")
+
+    @staticmethod
+    def _get_domain_from_id(mxid):
+        """Parses a string and returns the domain part of the mxid.
+
+        Args:
+           mxid (str): a valid mxid
+
+        Returns:
+           str: the domain part of the mxid
+
+        """
+        idx = mxid.find(":")
+        if idx == -1:
+            raise Exception("Invalid ID: %r" % (mxid,))
+        return mxid[idx + 1:]
diff --git a/tests/rulecheck/__init__.py b/tests/rulecheck/__init__.py
new file mode 100644
index 0000000000..a354d38ca8
--- /dev/null
+++ b/tests/rulecheck/__init__.py
@@ -0,0 +1,14 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
diff --git a/tests/rulecheck/test_domainrulecheck.py b/tests/rulecheck/test_domainrulecheck.py
new file mode 100644
index 0000000000..a24fb53766
--- /dev/null
+++ b/tests/rulecheck/test_domainrulecheck.py
@@ -0,0 +1,101 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+from tests import unittest
+
+from synapse.config._base import ConfigError
+from synapse.rulecheck.domain_rule_checker import DomainRuleChecker
+
+
+class DomainRuleCheckerTestCase(unittest.TestCase):
+
+    def test_allowed(self):
+        config = {
+            "default": False,
+            "domain_mapping": {
+                "source_one": ["target_one", "target_two"],
+                "source_two": ["target_two"]
+            }
+        }
+        check = DomainRuleChecker(config)
+        self.assertTrue(check.user_may_invite("test:source_one",
+                                              "test:target_one", "room"))
+        self.assertTrue(check.user_may_invite("test:source_one",
+                                              "test:target_two", "room"))
+        self.assertTrue(check.user_may_invite("test:source_two",
+                                              "test:target_two", "room"))
+
+    def test_disallowed(self):
+        config = {
+            "default": True,
+            "domain_mapping": {
+                "source_one": ["target_one", "target_two"],
+                "source_two": ["target_two"],
+                "source_four": []
+            }
+        }
+        check = DomainRuleChecker(config)
+        self.assertFalse(check.user_may_invite("test:source_one",
+                                               "test:target_three", "room"))
+        self.assertFalse(check.user_may_invite("test:source_two",
+                                               "test:target_three", "room"))
+        self.assertFalse(check.user_may_invite("test:source_two",
+                                               "test:target_one", "room"))
+        self.assertFalse(check.user_may_invite("test:source_four",
+                                               "test:target_one", "room"))
+
+    def test_default_allow(self):
+        config = {
+            "default": True,
+            "domain_mapping": {
+                "source_one": ["target_one", "target_two"],
+                "source_two": ["target_two"]
+            }
+        }
+        check = DomainRuleChecker(config)
+        self.assertTrue(check.user_may_invite("test:source_three",
+                                              "test:target_one", "room"))
+
+    def test_default_deny(self):
+        config = {
+            "default": False,
+            "domain_mapping": {
+                "source_one": ["target_one", "target_two"],
+                "source_two": ["target_two"]
+            }
+        }
+        check = DomainRuleChecker(config)
+        self.assertFalse(check.user_may_invite("test:source_three",
+                                               "test:target_one", "room"))
+
+    def test_config_parse(self):
+        config = {
+            "default": False,
+            "domain_mapping": {
+                "source_one": ["target_one", "target_two"],
+                "source_two": ["target_two"]
+            }
+        }
+        self.assertEquals(config, DomainRuleChecker.parse_config(config))
+
+    def test_config_parse_failure(self):
+        config = {
+            "domain_mapping": {
+                "source_one": ["target_one", "target_two"],
+                "source_two": ["target_two"]
+            }
+        }
+        self.assertRaises(ConfigError, DomainRuleChecker.parse_config, config)