diff --git a/src/__tests__/lib/exampleData.js b/src/__tests__/lib/exampleData.js
index 6027bc5bb3a..a5a05f65c6c 100644
--- a/src/__tests__/lib/exampleData.js
+++ b/src/__tests__/lib/exampleData.js
@@ -29,7 +29,7 @@ import type {
   MessagesState,
   RealmState,
 } from '../../reduxTypes';
-import type { Auth, Account, OutboxBase, StreamOutbox } from '../../types';
+import type { Auth, Account, StreamOutbox } from '../../types';
 import { UploadedAvatarURL } from '../../utils/avatar';
 import { ZulipVersion } from '../../utils/zulipVersion';
 import {
@@ -444,24 +444,18 @@ export const makeMessagesState = (messages: Message[]): MessagesState =>
  */
 
 /**
- * Properties in common among PM and stream outbox messages, with no
- *   interesting data.
+ * Boring properties common across example outbox messages.
  */
-const outboxMessageBase: $Diff<OutboxBase, {| id: mixed, timestamp: mixed |}> = deepFreeze({
+const outboxMessageBase = deepFreeze({
   isOutbox: true,
   isSent: false,
   avatar_url: selfUser.avatar_url,
   content: '<p>Test.</p>',
-  display_recipient: stream.name,
-  // id: ...,
   markdownContent: 'Test.',
   reactions: [],
   sender_email: selfUser.email,
   sender_full_name: selfUser.full_name,
   sender_id: selfUser.user_id,
-  subject: 'test topic',
-  // timestamp: ...,
-  type: 'stream',
 });
 
 /**
@@ -470,12 +464,16 @@ const outboxMessageBase: $Diff<OutboxBase, {| id: mixed, timestamp: mixed |}> =
  *
  * `.id` is always identical to `.timestamp` and should not be supplied.
  */
-export const streamOutbox = (data: $Shape<$Diff<StreamOutbox, {| id: mixed |}>>): StreamOutbox => {
+export const streamOutbox = (data: $Rest<StreamOutbox, { id: mixed, ... }>): StreamOutbox => {
   const { timestamp } = data;
 
   const outputTimestamp = timestamp ?? makeTime() / 1000;
   return deepFreeze({
     ...outboxMessageBase,
+    type: 'stream',
+    display_recipient: stream.name,
+    subject: 'test topic',
+
     ...data,
     id: outputTimestamp,
     timestamp: outputTimestamp,
diff --git a/src/outbox/outboxActions.js b/src/outbox/outboxActions.js
index 9efcc393641..8833fd33e21 100644
--- a/src/outbox/outboxActions.js
+++ b/src/outbox/outboxActions.js
@@ -9,6 +9,8 @@ import type {
   GlobalState,
   Narrow,
   Outbox,
+  PmOutbox,
+  StreamOutbox,
   UserOrBot,
   UserId,
   Action,
@@ -124,10 +126,9 @@ const recipientsFromIds = (ids, allUsersById, ownUser) => {
   return result;
 };
 
-type DataFromNarrow = SubsetProperties<
-  Outbox,
-  {| type: mixed, display_recipient: mixed, subject: mixed |},
->;
+type DataFromNarrow =
+  | SubsetProperties<PmOutbox, {| type: mixed, display_recipient: mixed, subject: mixed |}>
+  | SubsetProperties<StreamOutbox, {| type: mixed, display_recipient: mixed, subject: mixed |}>;
 
 const extractTypeToAndSubjectFromNarrow = (
   narrow: Narrow,
diff --git a/src/outbox/outboxReducer.js b/src/outbox/outboxReducer.js
index 87143987ce1..82ab0199b81 100644
--- a/src/outbox/outboxReducer.js
+++ b/src/outbox/outboxReducer.js
@@ -31,8 +31,8 @@ export default (state: OutboxState = initialState, action: Action): OutboxState
       return messageSendStart(state, action);
 
     case MESSAGE_SEND_COMPLETE:
-      return state.map(item =>
-        item.id !== action.local_message_id ? item : { ...item, isSent: true },
+      return state.map(<O: Outbox>(item: O) =>
+        item.id !== action.local_message_id ? item : { ...(item: O), isSent: true },
       );
 
     case DELETE_OUTBOX_MESSAGE:
diff --git a/src/types.js b/src/types.js
index 6d22c1bb586..0efc7a7e99c 100644
--- a/src/types.js
+++ b/src/types.js
@@ -167,12 +167,7 @@ export type TopicExtended = {|
  * Properties in common among the two different flavors of a
  * `Outbox`: `PmOutbox` and `StreamOutbox`.
  */
-// TODO: This distinction between `PmOutbox` and `StreamOutbox` doesn't yet
-//   function as fully as `PmMessage` and `StreamMessage`: for properties
-//   like `type` and `display_recipient` where the types differ between
-//   `PmMessage` and `StreamMessage`, it currently loses the information
-//   that those differences are connected to each other.
-export type OutboxBase = $ReadOnly<{|
+type OutboxBase = $ReadOnly<{|
   /** Used for distinguishing from a `Message` object. */
   isOutbox: true,
 
@@ -196,20 +191,14 @@ export type OutboxBase = $ReadOnly<{|
     // `Message` but potentially separately.
     Message,
     {|
-      // TODO: Some of these have different types on `PmMessage` vs.
-      //   `StreamMessage`; move those to `PmOutbox` and `StreamOutbox`
-      //   respectively, to match that distinction here.
       avatar_url: mixed,
       content: mixed,
-      display_recipient: mixed,
       id: mixed,
       reactions: mixed,
       sender_id: mixed,
       sender_email: mixed,
       sender_full_name: mixed,
-      subject: mixed,
       timestamp: mixed,
-      type: mixed,
     |},
   >,
 |}>;
@@ -217,7 +206,14 @@ export type OutboxBase = $ReadOnly<{|
 export type PmOutbox = $ReadOnly<{|
   ...OutboxBase,
 
-  ...SubsetProperties<PmMessage, {||}>,
+  ...SubsetProperties<
+    PmMessage,
+    {|
+      type: mixed,
+      display_recipient: mixed,
+      subject: mixed,
+    |},
+  >,
 |}>;
 
 export type StreamOutbox = $ReadOnly<{|
@@ -230,7 +226,14 @@ export type StreamOutbox = $ReadOnly<{|
   //   argument passed to `SubsetProperties` of `StreamMessage`, below.
   stream_id?: number,
 
-  ...SubsetProperties<StreamMessage, {||}>,
+  ...SubsetProperties<
+    StreamMessage,
+    {|
+      type: mixed,
+      display_recipient: mixed,
+      subject: mixed,
+    |},
+  >,
 |}>;
 
 /**
diff --git a/src/utils/recipient.js b/src/utils/recipient.js
index 5da1cbb2357..9dc3bee2142 100644
--- a/src/utils/recipient.js
+++ b/src/utils/recipient.js
@@ -1,5 +1,4 @@
 /* @flow strict-local */
-import invariant from 'invariant';
 // $FlowFixMe[untyped-import]
 import isEqual from 'lodash.isequal';
 
@@ -12,9 +11,7 @@ export const streamNameOfStreamMessage = (message: Message | Outbox): string =>
   if (message.type !== 'stream') {
     throw new Error('streamNameOfStreamMessage: got PM');
   }
-  const { display_recipient: streamName } = message;
-  invariant(typeof streamName === 'string', 'message type / display_recipient mismatch');
-  return streamName;
+  return message.display_recipient;
 };
 
 /** The recipients of a PM, in the form found on Message.  Throws if a stream message. */
@@ -24,9 +21,7 @@ export const recipientsOfPrivateMessage = (
   if (message.type !== 'private') {
     throw new Error('recipientsOfPrivateMessage: got stream message');
   }
-  const { display_recipient: recipients } = message;
-  invariant(typeof recipients === 'object', 'message type / display_recipient mismatch');
-  return recipients;
+  return message.display_recipient;
 };
 
 /**