diff --git a/api/client-server/registration.yaml b/api/client-server/registration.yaml
index e5875225..f2e50390 100644
--- a/api/client-server/registration.yaml
+++ b/api/client-server/registration.yaml
@@ -132,9 +132,71 @@ paths:
             "$ref": "definitions/error.yaml"
       tags:
         - User data
+  "/register/email/requestToken":
+    post:
+      summary: Requests a validation token be sent to the given email address
+      description: |-
+        Proxies the identity server API ``validate/email/requestToken``, but
+        first checks that the given email address is not already associated
+        with an account on this Home Server. Note that, for consistency,
+        this API takes JSON objects, though the Identity Server API takes
+        ``x-www-form-urlencoded`` parameters. See the Identity Server API for
+        further information.
+      parameters:
+        - in: body
+          name: body
+          schema:
+            type: object
+            properties:
+              id_server:
+                type: string
+                description: The ID server to send the onward request to as a hostname with an appended colon and port number if the port is not the default.
+                example: "id.matrix.org"
+              client_secret:
+                type: string
+                description: Client-generated secret string used to protect this session
+                example: "this_is_my_secret_string"
+              email:
+                type: string
+                description: The email address
+                example: "example@example.com"
+              send_attempt:
+                type: number
+                description: Used to distinguish protocol level retries from requests to re-send the email.
+                example: "1"
+            required: ["client_secret", "email", "send_attempt"]
+      responses:
+        200:
+          description: |-
+            An email has been sent to the specified address.
+            Note that this may be an email containing the validation token or it may be informing
+            the user of an error.
+          examples:
+            application/json: "{}"
+          schema:
+            type: object
+        400:
+          description: |-
+            Part of the request was invalid. This may include one of the following error codes:
+
+            * ``M_THREEPID_IN_USE`` : The email address is already registered to an account on this server.
+              However, if the home server has the ability to send email, it is recommended that the server
+              instead send an email to the user with instructions on how to reset their password.
+              This prevents malicious parties from being able to determine if a given email address
+              has an account on the Home Server in question.
+            * ``M_SERVER_NOT_TRUSTED`` : The ``id_server`` parameter refers to an ID server
+              that is not trusted by this Home Server.
+          examples:
+            application/json: |-
+              {
+                "errcode": "M_THREEPID_IN_USE",
+                "error": "The specified address is already in use"
+              }
+          schema:
+            type: object
   "/account/password":
     post:
-      summary: Changes a user's password.
+      summary: "Changes a user's password."
       description: |-
         Changes the password for an account on this homeserver.
 
diff --git a/specification/client_server_api.rst b/specification/client_server_api.rst
index dd93f774..1e9feaff 100644
--- a/specification/client_server_api.rst
+++ b/specification/client_server_api.rst
@@ -113,6 +113,12 @@ Some requests have unique error codes:
 :``M_BAD_PAGINATION``:
   Encountered when specifying bad pagination query parameters.
 
+:``M_THREEPID_IN_USE``:
+  Sent when a threepid given to an API cannot be used because the same threepid is already in use.
+
+:``M_SERVER_NOT_TRUSTED``:
+  The client's request used a third party server, eg. ID server, that this server does not trust.
+
 .. _sect:txn_ids:
 
 The client-server API typically uses ``HTTP PUT`` to submit requests with a
diff --git a/specification/identity_service_api.rst b/specification/identity_service_api.rst
index 0aa34343..d6f5d324 100644
--- a/specification/identity_service_api.rst
+++ b/specification/identity_service_api.rst
@@ -124,6 +124,11 @@ This is to avoid repeatedly sending the same email in the case of request
 retries between the POSTing user and the identity service. The client should
 increment this value if they desire a new email (e.g. a reminder) to be sent.
 
+Note that Home Servers offer APIs that proxy this API, adding additional
+behaviour on top, for example, ``/register/email/requestToken`` is designed
+specifically for use when registering an account and therefore will inform
+the user if the email address given is already registered on the server.
+
 Validating ownership of an email
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~