diff --git a/synapse/python_dependencies.py b/synapse/python_dependencies.py

index f64baa4d58..6efd81f204 100644
--- a/synapse/python_dependencies.py
+++ b/synapse/python_dependencies.py
@@ -44,7 +44,10 @@ REQUIREMENTS = [
     "canonicaljson>=1.1.3",
     "signedjson>=1.0.0",
     "pynacl>=1.2.1",
-    "service_identity>=16.0.0",
+    "idna>=2",
+
+    # validating SSL certs for IP addresses requires service_identity 18.1.
+    "service_identity>=18.1.0",
 
     # our logcontext handling relies on the ability to cancel inlineCallbacks
     # (https://twistedmatrix.com/trac/ticket/4632) which landed in Twisted 18.7.
@@ -77,7 +80,7 @@ REQUIREMENTS = [
 ]
 
 CONDITIONAL_REQUIREMENTS = {
-    "email.enable_notifs": ["Jinja2>=2.9", "bleach>=1.4.2"],
+    "email": ["Jinja2>=2.9", "bleach>=1.4.2"],
     "matrix-synapse-ldap3": ["matrix-synapse-ldap3>=0.1"],
 
     # we use execute_batch, which arrived in psycopg 2.7.