diff --git a/synapse/rest/client/v1/login.py b/synapse/rest/client/v1/login.py
index 093bc072f4..0c9cdff3b8 100644
--- a/synapse/rest/client/v1/login.py
+++ b/synapse/rest/client/v1/login.py
@@ -118,8 +118,14 @@ class LoginRestServlet(ClientV1RestServlet):
     @defer.inlineCallbacks
     def do_password_login(self, login_submission):
         if 'medium' in login_submission and 'address' in login_submission:
+            address = login_submission['address']
+            if login_submission['medium'] == 'email':
+                # For emails, transform the address to lowercase.
+                # We store all email addreses as lowercase in the DB.
+                # (See add_threepid in synapse/handlers/auth.py)
+                address = address.lower()
             user_id = yield self.hs.get_datastore().get_user_id_by_threepid(
-                login_submission['medium'], login_submission['address']
+                login_submission['medium'], address
             )
             if not user_id:
                 raise LoginError(403, "", errcode=Codes.FORBIDDEN)
diff --git a/synapse/rest/client/v2_alpha/account.py b/synapse/rest/client/v2_alpha/account.py
index e74e5e0123..398e7f5eb0 100644
--- a/synapse/rest/client/v2_alpha/account.py
+++ b/synapse/rest/client/v2_alpha/account.py
@@ -96,6 +96,11 @@ class PasswordRestServlet(RestServlet):
             threepid = result[LoginType.EMAIL_IDENTITY]
             if 'medium' not in threepid or 'address' not in threepid:
                 raise SynapseError(500, "Malformed threepid")
+            if threepid['medium'] == 'email':
+                # For emails, transform the address to lowercase.
+                # We store all email addreses as lowercase in the DB.
+                # (See add_threepid in synapse/handlers/auth.py)
+                threepid['address'] = threepid['address'].lower()
             # if using email, we must know about the email they're authing with!
             threepid_user_id = yield self.hs.get_datastore().get_user_id_by_threepid(
                 threepid['medium'], threepid['address']