diff --git a/synapse/config/registration.py b/synapse/config/registration.py
index 6cd830a78e..dc3c85a517 100644
--- a/synapse/config/registration.py
+++ b/synapse/config/registration.py
@@ -36,6 +36,7 @@ class RegistrationConfig(Config):
         self.check_is_for_allowed_local_3pids = config.get(
             "check_is_for_allowed_local_3pids", False
         )
+        self.allow_invited_3pids = config.get("allow_invited_3pids", False)
         self.registration_shared_secret = config.get("registration_shared_secret")
 
         self.bcrypt_rounds = config.get("bcrypt_rounds", 12)
@@ -70,6 +71,12 @@ class RegistrationConfig(Config):
         # Overrides allowed_local_3pids below.
         # check_is_for_allowed_local_3pids: matrix.org
         #
+        # If you are using an IS you can also check whether that IS registers
+        # pending invites for the given 3PID (and then allow it to sign up on
+        # the platform):
+        #
+        # allow_invited_3pids: False
+        #
         # allowed_local_3pids:
         #     - medium: email
         #       pattern: ".*@matrix\\.org"
diff --git a/synapse/util/threepids.py b/synapse/util/threepids.py
index e1cf5dbb13..94c0852f0c 100644
--- a/synapse/util/threepids.py
+++ b/synapse/util/threepids.py
@@ -42,7 +42,10 @@ def check_3pid_allowed(hs, medium, address):
             ),
             {'medium': medium, 'address': address}
         )
-        defer.returnValue(data['hs_url'] + "/" == hs.config.public_baseurl)
+        if hs.config.allow_invited_3pids and data.get('invited'):
+            defer.returnValue(True)
+        else:
+            defer.returnValue(data['hs_url'] + "/" == hs.config.public_baseurl)
         return
 
     if hs.config.allowed_local_3pids: