diff --git a/src/Modal.js b/src/Modal.js
index 89e8b1361c..b6cc46ed45 100644
--- a/src/Modal.js
+++ b/src/Modal.js
@@ -177,7 +177,7 @@ class ModalManager {
 
         var modal = this._modals[0];
         var dialog = (
-            <div className={"mx_Dialog_wrapper " + modal.className}>
+            <div className={"mx_Dialog_wrapper " + (modal.className ? modal.className : '') }>
                 <div className="mx_Dialog">
                     {modal.elem}
                 </div>
diff --git a/src/Resend.js b/src/Resend.js
index ecf504e780..59a8bd4192 100644
--- a/src/Resend.js
+++ b/src/Resend.js
@@ -16,17 +16,26 @@ limitations under the License.
 
 var MatrixClientPeg = require('./MatrixClientPeg');
 var dis = require('./dispatcher');
+var sdk = require('./index');
+var Modal = require('./Modal');
 
 module.exports = {
     resend: function(event) {
         MatrixClientPeg.get().resendEvent(
             event, MatrixClientPeg.get().getRoom(event.getRoomId())
-        ).done(function() {
+        ).done(function(res) {
             dis.dispatch({
                 action: 'message_sent',
                 event: event
             });
-        }, function() {
+        }, function(err) {
+            if (err.name === "UnknownDeviceError") {
+                var UnknownDeviceDialog = sdk.getComponent("dialogs.UnknownDeviceDialog");
+                Modal.createDialog(UnknownDeviceDialog, {
+                    devices: err.devices
+                }, "mx_Dialog_unknownDevice");
+            }
+
             dis.dispatch({
                 action: 'message_send_failed',
                 event: event
diff --git a/src/component-index.js b/src/component-index.js
index c50ee0dfc8..5b28be0627 100644
--- a/src/component-index.js
+++ b/src/component-index.js
@@ -89,6 +89,8 @@ import views$dialogs$SetDisplayNameDialog from './components/views/dialogs/SetDi
 views$dialogs$SetDisplayNameDialog && (module.exports.components['views.dialogs.SetDisplayNameDialog'] = views$dialogs$SetDisplayNameDialog);
 import views$dialogs$TextInputDialog from './components/views/dialogs/TextInputDialog';
 views$dialogs$TextInputDialog && (module.exports.components['views.dialogs.TextInputDialog'] = views$dialogs$TextInputDialog);
+import views$dialogs$UnknownDeviceDialog from './components/views/dialogs/UnknownDeviceDialog';
+views$dialogs$UnknownDeviceDialog && (module.exports.components['views.dialogs.UnknownDeviceDialog'] = views$dialogs$UnknownDeviceDialog);
 import views$elements$AccessibleButton from './components/views/elements/AccessibleButton';
 views$elements$AccessibleButton && (module.exports.components['views.elements.AccessibleButton'] = views$elements$AccessibleButton);
 import views$elements$AddressSelector from './components/views/elements/AddressSelector';
diff --git a/src/components/views/dialogs/UnknownDeviceDialog.js b/src/components/views/dialogs/UnknownDeviceDialog.js
new file mode 100644
index 0000000000..be1138fe43
--- /dev/null
+++ b/src/components/views/dialogs/UnknownDeviceDialog.js
@@ -0,0 +1,108 @@
+/*
+Copyright 2017 Vector Creations Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import React from 'react';
+import sdk from '../../../index';
+import MatrixClientPeg from '../../../MatrixClientPeg';
+import GeminiScrollbar from 'react-gemini-scrollbar';
+
+function UserUnknownDeviceList(props) {
+    const {userDevices} = props;
+
+    const deviceListEntries = Object.keys(userDevices).map((deviceId) =>
+        <li key={ deviceId }>
+            { deviceId } ( { userDevices[deviceId].getDisplayName() } )
+        </li>,
+    );
+
+    return <ul>{deviceListEntries}</ul>;
+}
+
+UserUnknownDeviceList.propTypes = {
+    // map from deviceid -> deviceinfo
+    userDevices: React.PropTypes.object.isRequired,
+};
+
+
+function UnknownDeviceList(props) {
+    const {devices} = props;
+
+    const userListEntries = Object.keys(devices).map((userId) =>
+        <li key={ userId }>
+            <p>{ userId }:</p>
+            <UserUnknownDeviceList userDevices={devices[userId]} />
+        </li>,
+    );
+
+    return <ul>{userListEntries}</ul>;
+}
+
+UnknownDeviceList.propTypes = {
+    // map from userid -> deviceid -> deviceinfo
+    devices: React.PropTypes.object.isRequired,
+};
+
+
+export default React.createClass({
+    displayName: 'UnknownEventDialog',
+
+    propTypes: {
+        // map from userid -> deviceid -> deviceinfo
+        devices: React.PropTypes.object.isRequired,
+        onFinished: React.PropTypes.func.isRequired,
+    },
+
+    componentDidMount: function() {
+        // Given we've now shown the user the unknown device, it is no longer
+        // unknown to them. Therefore mark it as 'known'.
+        Object.keys(this.props.devices).forEach((userId) => {
+            Object.keys(this.props.devices[userId]).map((deviceId) => {
+                MatrixClientPeg.get().setDeviceKnown(userId, deviceId, true);
+            });
+        });
+    },
+
+    render: function() {
+        const BaseDialog = sdk.getComponent('views.dialogs.BaseDialog');
+        return (
+            <BaseDialog className='mx_UnknownDeviceDialog'
+                onFinished={this.props.onFinished}
+                title='Room contains unknown devices'
+            >
+                <GeminiScrollbar autoshow={false} className="mx_Dialog_content">
+                    <h4>This room contains devices which have not been
+                    verified.</h4>
+                    <p>
+                        This means there is no guarantee that the devices belong
+                        to a rightful user of the room.
+                    </p><p>
+                        We recommend you go through the verification process
+                        for each device before continuing, but you can resend
+                        the message without verifying if you prefer.
+                    </p>
+                    <p>Unknown devices:</p>
+                    <UnknownDeviceList devices={this.props.devices} />
+                </GeminiScrollbar>
+                <div className="mx_Dialog_buttons">
+                    <button className="mx_Dialog_primary" autoFocus={ true }
+                            onClick={ this.props.onFinished } >
+                        OK
+                    </button>
+                </div>
+            </BaseDialog>
+        );
+    },
+});
diff --git a/src/components/views/rooms/MessageComposerInput.js b/src/components/views/rooms/MessageComposerInput.js
index bf936a2c13..80e41555a2 100644
--- a/src/components/views/rooms/MessageComposerInput.js
+++ b/src/components/views/rooms/MessageComposerInput.js
@@ -40,6 +40,7 @@ import * as HtmlUtils from '../../../HtmlUtils';
 import Autocomplete from './Autocomplete';
 import {Completion} from "../../../autocomplete/Autocompleter";
 import Markdown from '../../../Markdown';
+import {onSendMessageFailed} from './MessageComposerInputOld';
 
 const TYPING_USER_TIMEOUT = 10000, TYPING_SERVER_TIMEOUT = 30000;
 
@@ -553,15 +554,11 @@ export default class MessageComposerInput extends React.Component {
             sendMessagePromise = sendTextFn.call(this.client, this.props.room.roomId, contentText);
         }
 
-        sendMessagePromise.then(() => {
+        sendMessagePromise.done((res) => {
             dis.dispatch({
                 action: 'message_sent',
             });
-        }, () => {
-            dis.dispatch({
-                action: 'message_send_failed',
-            });
-        });
+        }, onSendMessageFailed);
 
         this.setState({
             editorState: this.createEditorState(),
diff --git a/src/components/views/rooms/MessageComposerInputOld.js b/src/components/views/rooms/MessageComposerInputOld.js
index c5d5f083c1..e8a4727fc6 100644
--- a/src/components/views/rooms/MessageComposerInputOld.js
+++ b/src/components/views/rooms/MessageComposerInputOld.js
@@ -29,10 +29,22 @@ var TYPING_USER_TIMEOUT = 10000;
 var TYPING_SERVER_TIMEOUT = 30000;
 var MARKDOWN_ENABLED = true;
 
+export function onSendMessageFailed(err) {
+    if (err.name === "UnknownDeviceError") {
+        const UnknownDeviceDialog = sdk.getComponent("dialogs.UnknownDeviceDialog");
+        Modal.createDialog(UnknownDeviceDialog, {
+            devices: err.devices,
+        }, "mx_Dialog_unknownDevice");
+    }
+    dis.dispatch({
+        action: 'message_send_failed',
+    });
+}
+
 /*
  * The textInput part of the MessageComposer
  */
-module.exports = React.createClass({
+export default React.createClass({
     displayName: 'MessageComposerInput',
 
     statics: {
@@ -337,15 +349,12 @@ module.exports = React.createClass({
                 MatrixClientPeg.get().sendTextMessage(this.props.room.roomId, contentText);
         }
 
-        sendMessagePromise.done(function() {
+        sendMessagePromise.done(function(res) {
             dis.dispatch({
                 action: 'message_sent'
             });
-        }, function() {
-            dis.dispatch({
-                action: 'message_send_failed'
-            });
-        });
+        }, onSendMessageFailed);
+
         this.refs.textarea.value = '';
         this.resizeInput();
         ev.preventDefault();