diff --git a/synapse/rest/client/v2_alpha/account.py b/synapse/rest/client/v2_alpha/account.py
index 372648caf..37b32dd37 100644
--- a/synapse/rest/client/v2_alpha/account.py
+++ b/synapse/rest/client/v2_alpha/account.py
@@ -53,7 +53,9 @@ class EmailPasswordRequestTokenRestServlet(RestServlet):
 
         if not check_3pid_allowed(self.hs, "email", body['email']):
             raise SynapseError(
-                403, "Third party identifier is not allowed", Codes.THREEPID_DENIED,
+                403,
+                "Your email domain is not authorized on this server",
+                Codes.THREEPID_DENIED,
             )
 
         existingUid = yield self.hs.get_datastore().get_user_id_by_threepid(
@@ -89,7 +91,9 @@ class MsisdnPasswordRequestTokenRestServlet(RestServlet):
 
         if not check_3pid_allowed(self.hs, "msisdn", msisdn):
             raise SynapseError(
-                403, "Third party identifier is not allowed", Codes.THREEPID_DENIED,
+                403,
+                "Account phone numbers are not authorized on this server",
+                Codes.THREEPID_DENIED,
             )
 
         existingUid = yield self.datastore.get_user_id_by_threepid(
@@ -241,7 +245,9 @@ class EmailThreepidRequestTokenRestServlet(RestServlet):
 
         if not check_3pid_allowed(self.hs, "email", body['email']):
             raise SynapseError(
-                403, "Third party identifier is not allowed", Codes.THREEPID_DENIED,
+                403,
+                "Your email domain is not authorized on this server",
+                Codes.THREEPID_DENIED,
             )
 
         existingUid = yield self.datastore.get_user_id_by_threepid(
@@ -276,7 +282,9 @@ class MsisdnThreepidRequestTokenRestServlet(RestServlet):
 
         if not check_3pid_allowed(self.hs, "msisdn", msisdn):
             raise SynapseError(
-                403, "Third party identifier is not allowed", Codes.THREEPID_DENIED,
+                403,
+                "Account phone numbers are not authorized on this server",
+                Codes.THREEPID_DENIED,
             )
 
         existingUid = yield self.datastore.get_user_id_by_threepid(
diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index 2fb4d43cc..200b29fa3 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -75,7 +75,9 @@ class EmailRegisterRequestTokenRestServlet(RestServlet):
 
         if not check_3pid_allowed(self.hs, "email", body['email']):
             raise SynapseError(
-                403, "Third party identifier is not allowed", Codes.THREEPID_DENIED,
+                403,
+                "Your email domain is not authorized to register on this server",
+                Codes.THREEPID_DENIED,
             )
 
         existingUid = yield self.hs.get_datastore().get_user_id_by_threepid(
@@ -115,7 +117,9 @@ class MsisdnRegisterRequestTokenRestServlet(RestServlet):
 
         if not check_3pid_allowed(self.hs, "msisdn", msisdn):
             raise SynapseError(
-                403, "Third party identifier is not allowed", Codes.THREEPID_DENIED,
+                403,
+                "Phone numbers are not authorized to register on this server",
+                Codes.THREEPID_DENIED,
             )
 
         existingUid = yield self.hs.get_datastore().get_user_id_by_threepid(
@@ -373,7 +377,8 @@ class RegisterRestServlet(RestServlet):
 
                     if not check_3pid_allowed(self.hs, medium, address):
                         raise SynapseError(
-                            403, "Third party identifier is not allowed",
+                            403,
+                            "Third party identifier (email/phone number) is not allowed",
                             Codes.THREEPID_DENIED,
                         )