diff --git a/Podfile.lock b/Podfile.lock
index 69667daec..a23e3c1a8 100644
--- a/Podfile.lock
+++ b/Podfile.lock
@@ -268,7 +268,7 @@ CHECKOUT OPTIONS:
     :commit: b60dc7d58dfc93ca6eafbb3ea5300c6d67ebc69a
     :git: https://github.com/signalapp/SignalCoreKit.git
   SignalMetadataKit:
-    :commit: b493b7b0db57902dc766a50ca97d2202453ba512
+    :commit: 56f28fc3a6e35d548d034ef7d0009f233ca0aa62
     :git: https://github.com/signalapp/SignalMetadataKit
   SocketRocket:
     :commit: 9f9563a83cd8960503074aa8de72206f83fb7a69
diff --git a/Pods b/Pods
index 5fface14b..afb050136 160000
--- a/Pods
+++ b/Pods
@@ -1 +1 @@
-Subproject commit 5fface14b1bed97e4b7686947f1a24953700ac26
+Subproject commit afb050136acfdaec83e2e88e25f56db9af771e2c
diff --git a/SignalServiceKit/src/Messages/OWSMessageManager.m b/SignalServiceKit/src/Messages/OWSMessageManager.m
index 79b4a1e04..1ae6b2805 100644
--- a/SignalServiceKit/src/Messages/OWSMessageManager.m
+++ b/SignalServiceKit/src/Messages/OWSMessageManager.m
@@ -537,12 +537,18 @@ NS_ASSUME_NONNULL_BEGIN
     }
 
     // Send delivery receipts for "valid data" messages received via UD.
-    BOOL wasReceivedByUD = envelope.type == SSKProtoEnvelopeTypeUnidentifiedSender;
+    BOOL wasReceivedByUD = [self wasReceivedByUD:envelope];
     if (wasReceivedByUD) {
         [self.outgoingReceiptManager enqueueDeliveryReceiptForEnvelope:envelope];
     }
 }
 
+- (BOOL)wasReceivedByUD:(SSKProtoEnvelope *)envelope
+{
+    return (
+        envelope.type == SSKProtoEnvelopeTypeUnidentifiedSender && (!envelope.hasSource || envelope.source.length < 1));
+}
+
 - (void)sendGroupInfoRequest:(NSData *)groupId
                     envelope:(SSKProtoEnvelope *)envelope
                  transaction:(YapDatabaseReadWriteTransaction *)transaction
@@ -1149,7 +1155,7 @@ NS_ASSUME_NONNULL_BEGIN
     NSData *groupId = dataMessage.group ? dataMessage.group.id : nil;
     OWSContact *_Nullable contact = [OWSContacts contactForDataMessage:dataMessage transaction:transaction];
     NSNumber *_Nullable serverTimestamp = (envelope.hasServerTimestamp ? @(envelope.serverTimestamp) : nil);
-    BOOL wasReceivedByUD = envelope.type == SSKProtoEnvelopeTypeUnidentifiedSender;
+    BOOL wasReceivedByUD = [self wasReceivedByUD:envelope];
 
     if (dataMessage.group.type == SSKProtoGroupContextTypeRequestInfo) {
         [self handleGroupInfoRequest:envelope dataMessage:dataMessage transaction:transaction];
diff --git a/SignalServiceKit/src/Messages/OWSMessageSender.m b/SignalServiceKit/src/Messages/OWSMessageSender.m
index cc1307622..ea0182de1 100644
--- a/SignalServiceKit/src/Messages/OWSMessageSender.m
+++ b/SignalServiceKit/src/Messages/OWSMessageSender.m
@@ -1044,6 +1044,25 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
         }
     }
 
+    for (NSDictionary *deviceMessage in deviceMessages) {
+        NSNumber *_Nullable messageType = deviceMessage[@"type"];
+        OWSAssertDebug(messageType);
+        BOOL hasValidMessageType;
+        if (messageSend.isUDSend) {
+            hasValidMessageType = [messageType isEqualToNumber:@(TSUnidentifiedSenderMessageType)];
+        } else {
+            hasValidMessageType = ([messageType isEqualToNumber:@(TSEncryptedWhisperMessageType)] ||
+                [messageType isEqualToNumber:@(TSPreKeyWhisperMessageType)]);
+        }
+
+        if (!hasValidMessageType) {
+            OWSFailDebug(@"Invalid message type: %@", messageType);
+            NSError *error = OWSErrorMakeFailedToSendOutgoingMessageError();
+            [error setIsRetryable:NO];
+            return messageSend.failure(error);
+        }
+    }
+
     if (deviceMessages.count == 0) {
         // This might happen:
         //
@@ -1061,15 +1080,6 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
         OWSLogWarn(@"Sending a message with no device messages.");
     }
 
-    // NOTE: canFailoverUDAuth depends on whether or not we're sending a
-    // sync message because sync messages use different device lists
-    // for UD-auth and Non-UD-auth requests.
-    //
-    // Therefore, for sync messages, we can't use OWSRequestMaker's
-    // retry/failover logic; we need to use the message sender's retry
-    // logic that will build a new set of device messages.
-    BOOL isSyncMessageSend = messageSend.isLocalNumber;
-    BOOL canFailoverUDAuth = !isSyncMessageSend;
     OWSRequestMaker *requestMaker = [[OWSRequestMaker alloc] initWithLabel:@"Message Send"
         requestFactoryBlock:^(SMKUDAccessKey *_Nullable udAccessKey) {
             return [OWSRequestFactory submitMessageRequestWithRecipient:recipient.recipientId
@@ -1089,7 +1099,7 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
         }
         recipientId:recipient.recipientId
         udAccess:messageSend.udAccess
-        canFailoverUDAuth:canFailoverUDAuth];
+        canFailoverUDAuth:NO];
     [[requestMaker makeRequestObjc]
             .then(^(OWSRequestMakerResult *result) {
                 dispatch_async([OWSDispatch sendingQueue], ^{
@@ -1437,7 +1447,7 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
             // we open a transaction.
             [self throws_ensureRecipientHasSessionForMessageSend:messageSend deviceId:deviceId];
 
-            __block NSDictionary *messageDict;
+            __block NSDictionary *_Nullable messageDict;
             __block NSException *encryptionException;
             [self.dbConnection
                 readWriteWithBlock:^(YapDatabaseReadWriteTransaction *transaction) {
@@ -1605,10 +1615,10 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
             }) retainUntilComplete];
 }
 
-- (NSDictionary *)throws_encryptedMessageForMessageSend:(OWSMessageSend *)messageSend
-                                               deviceId:(NSNumber *)deviceId
-                                              plainText:(NSData *)plainText
-                                            transaction:(YapDatabaseReadWriteTransaction *)transaction
+- (nullable NSDictionary *)throws_encryptedMessageForMessageSend:(OWSMessageSend *)messageSend
+                                                        deviceId:(NSNumber *)deviceId
+                                                       plainText:(NSData *)plainText
+                                                     transaction:(YapDatabaseReadWriteTransaction *)transaction
 {
     OWSAssertDebug(messageSend);
     OWSAssertDebug(deviceId);
@@ -1658,6 +1668,10 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
                                                                       protocolContext:transaction
                                                                                 error:&error];
         SCKRaiseIfExceptionWrapperError(error);
+        if (!serializedMessage || error) {
+            OWSFailDebug(@"error while UD encrypting message: %@", error);
+            return nil;
+        }
         messageType = TSUnidentifiedSenderMessageType;
     } else {
         // This may throw an exception.
diff --git a/SignalServiceKit/src/Network/API/TSNetworkManager.m b/SignalServiceKit/src/Network/API/TSNetworkManager.m
index 94e7448f9..9af915388 100644
--- a/SignalServiceKit/src/Network/API/TSNetworkManager.m
+++ b/SignalServiceKit/src/Network/API/TSNetworkManager.m
@@ -27,6 +27,7 @@ BOOL IsNSErrorNetworkFailure(NSError *_Nullable error)
 
 // This property should only be accessed on udSerialQueue.
 @property (atomic, readonly) AFHTTPSessionManager *udSessionManager;
+@property (atomic, readonly) NSDictionary *udSessionManagerDefaultHeaders;
 
 @property (atomic, readonly) dispatch_queue_t udSerialQueue;
 
@@ -139,9 +140,12 @@ typedef void (^failureBlock)(NSURLSessionDataTask *task, NSError *error);
 - (AFHTTPSessionManager *)udSessionManager
 {
     if (!_udSessionManager) {
-        _udSessionManager = [OWSSignalService sharedInstance].signalServiceSessionManager;
-        _udSessionManager.completionQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
+        AFHTTPSessionManager *udSessionManager = [OWSSignalService sharedInstance].signalServiceSessionManager;
+        udSessionManager.completionQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
         // NOTE: We could enable HTTPShouldUsePipelining here.
+        _udSessionManager = udSessionManager;
+        // Make a copy of the default headers for this session manager.
+        _udSessionManagerDefaultHeaders = [udSessionManager.requestSerializer.HTTPRequestHeaders copy];
     }
 
     return _udSessionManager;
@@ -169,10 +173,16 @@ typedef void (^failureBlock)(NSURLSessionDataTask *task, NSError *error);
 
     AFHTTPSessionManager *sessionManager = self.udSessionManager;
 
-    // Honor the request's headers.
+    // Clear all headers so that we don't retain headers from previous requests.
     for (NSString *headerField in sessionManager.requestSerializer.HTTPRequestHeaders.allKeys.copy) {
         [sessionManager.requestSerializer setValue:nil forHTTPHeaderField:headerField];
     }
+    // Apply the default headers for this session manager.
+    for (NSString *headerField in self.udSessionManagerDefaultHeaders) {
+        NSString *headerValue = self.udSessionManagerDefaultHeaders[headerField];
+        [sessionManager.requestSerializer setValue:headerValue forHTTPHeaderField:headerField];
+    }
+    // Honor the request's headers.
     for (NSString *headerField in request.allHTTPHeaderFields) {
         NSString *headerValue = request.allHTTPHeaderFields[headerField];
         [sessionManager.requestSerializer setValue:headerValue forHTTPHeaderField:headerField];