diff --git a/amazonka-sqs.cabal b/amazonka-sqs.cabal
--- a/amazonka-sqs.cabal
+++ b/amazonka-sqs.cabal
@@ -1,5 +1,5 @@
 name:                  amazonka-sqs
-version:               0.0.0
+version:               0.0.1
 synopsis:              Amazon Simple Queue Service SDK.
 homepage:              https://github.com/brendanhay/amazonka
 license:               OtherLicense
@@ -60,5 +60,5 @@
     other-modules:
 
     build-depends:
-          amazonka-core
-        , base          >= 4.7 && < 5
+          amazonka-core == 0.0.1.*
+        , base          >= 4.7     && < 5
diff --git a/gen/Network/AWS/SQS/AddPermission.hs b/gen/Network/AWS/SQS/AddPermission.hs
--- a/gen/Network/AWS/SQS/AddPermission.hs
+++ b/gen/Network/AWS/SQS/AddPermission.hs
@@ -72,14 +72,12 @@
 --
 addPermission :: Text -- ^ 'apQueueUrl'
               -> Text -- ^ 'apLabel'
-              -> [Text] -- ^ 'apAWSAccountIds'
-              -> [Text] -- ^ 'apActions'
               -> AddPermission
-addPermission p1 p2 p3 p4 = AddPermission
+addPermission p1 p2 = AddPermission
     { _apQueueUrl      = p1
     , _apLabel         = p2
-    , _apAWSAccountIds = withIso _List (const id) p3
-    , _apActions       = withIso _List (const id) p4
+    , _apAWSAccountIds = mempty
+    , _apActions       = mempty
     }
 
 -- | The AWS account number of the principal who will be given permission. The
diff --git a/gen/Network/AWS/SQS/ChangeMessageVisibilityBatch.hs b/gen/Network/AWS/SQS/ChangeMessageVisibilityBatch.hs
--- a/gen/Network/AWS/SQS/ChangeMessageVisibilityBatch.hs
+++ b/gen/Network/AWS/SQS/ChangeMessageVisibilityBatch.hs
@@ -68,11 +68,10 @@
 -- * 'cmvbQueueUrl' @::@ 'Text'
 --
 changeMessageVisibilityBatch :: Text -- ^ 'cmvbQueueUrl'
-                             -> [ChangeMessageVisibilityBatchRequestEntry] -- ^ 'cmvbEntries'
                              -> ChangeMessageVisibilityBatch
-changeMessageVisibilityBatch p1 p2 = ChangeMessageVisibilityBatch
+changeMessageVisibilityBatch p1 = ChangeMessageVisibilityBatch
     { _cmvbQueueUrl = p1
-    , _cmvbEntries  = withIso _List (const id) p2
+    , _cmvbEntries  = mempty
     }
 
 -- | A list of receipt handles of the messages for which the visibility
@@ -97,12 +96,10 @@
 --
 -- * 'cmvbrSuccessful' @::@ ['ChangeMessageVisibilityBatchResultEntry']
 --
-changeMessageVisibilityBatchResponse :: [ChangeMessageVisibilityBatchResultEntry] -- ^ 'cmvbrSuccessful'
-                                     -> [BatchResultErrorEntry] -- ^ 'cmvbrFailed'
-                                     -> ChangeMessageVisibilityBatchResponse
-changeMessageVisibilityBatchResponse p1 p2 = ChangeMessageVisibilityBatchResponse
-    { _cmvbrSuccessful = withIso _List (const id) p1
-    , _cmvbrFailed     = withIso _List (const id) p2
+changeMessageVisibilityBatchResponse :: ChangeMessageVisibilityBatchResponse
+changeMessageVisibilityBatchResponse = ChangeMessageVisibilityBatchResponse
+    { _cmvbrSuccessful = mempty
+    , _cmvbrFailed     = mempty
     }
 
 -- | A list of BatchResultErrorEntry items.
diff --git a/gen/Network/AWS/SQS/CreateQueue.hs b/gen/Network/AWS/SQS/CreateQueue.hs
--- a/gen/Network/AWS/SQS/CreateQueue.hs
+++ b/gen/Network/AWS/SQS/CreateQueue.hs
@@ -57,7 +57,7 @@
 import qualified GHC.Exts
 
 data CreateQueue = CreateQueue
-    { _cqAttributes :: EMap "Attribute" "Name" "Value" Text Text
+    { _cqAttributes :: EMap "Attribute" "Name" "Value" QueueAttributeName Text
     , _cqQueueName  :: Text
     } deriving (Eq, Show)
 
@@ -65,16 +65,15 @@
 --
 -- The fields accessible through corresponding lenses are:
 --
--- * 'cqAttributes' @::@ 'HashMap' 'Text' 'Text'
+-- * 'cqAttributes' @::@ 'HashMap' 'QueueAttributeName' 'Text'
 --
 -- * 'cqQueueName' @::@ 'Text'
 --
 createQueue :: Text -- ^ 'cqQueueName'
-            -> HashMap Text Text -- ^ 'cqAttributes'
             -> CreateQueue
-createQueue p1 p2 = CreateQueue
+createQueue p1 = CreateQueue
     { _cqQueueName  = p1
-    , _cqAttributes = withIso _EMap (const id) p2
+    , _cqAttributes = mempty
     }
 
 -- | A map of attributes with their corresponding values. The following lists
@@ -98,7 +97,7 @@
 -- timeout for the queue. An integer from 0 to 43200 (12 hours). The default
 -- for this attribute is 30. For more information about visibility timeout,
 -- see Visibility Timeout in the Amazon SQS Developer Guide.
-cqAttributes :: Lens' CreateQueue (HashMap Text Text)
+cqAttributes :: Lens' CreateQueue (HashMap QueueAttributeName Text)
 cqAttributes = lens _cqAttributes (\s a -> s { _cqAttributes = a }) . _EMap
 
 -- | The name for the queue to be created.
diff --git a/gen/Network/AWS/SQS/DeleteMessageBatch.hs b/gen/Network/AWS/SQS/DeleteMessageBatch.hs
--- a/gen/Network/AWS/SQS/DeleteMessageBatch.hs
+++ b/gen/Network/AWS/SQS/DeleteMessageBatch.hs
@@ -66,11 +66,10 @@
 -- * 'dmbQueueUrl' @::@ 'Text'
 --
 deleteMessageBatch :: Text -- ^ 'dmbQueueUrl'
-                   -> [DeleteMessageBatchRequestEntry] -- ^ 'dmbEntries'
                    -> DeleteMessageBatch
-deleteMessageBatch p1 p2 = DeleteMessageBatch
+deleteMessageBatch p1 = DeleteMessageBatch
     { _dmbQueueUrl = p1
-    , _dmbEntries  = withIso _List (const id) p2
+    , _dmbEntries  = mempty
     }
 
 -- | A list of receipt handles for the messages to be deleted.
@@ -94,12 +93,10 @@
 --
 -- * 'dmbrSuccessful' @::@ ['DeleteMessageBatchResultEntry']
 --
-deleteMessageBatchResponse :: [DeleteMessageBatchResultEntry] -- ^ 'dmbrSuccessful'
-                           -> [BatchResultErrorEntry] -- ^ 'dmbrFailed'
-                           -> DeleteMessageBatchResponse
-deleteMessageBatchResponse p1 p2 = DeleteMessageBatchResponse
-    { _dmbrSuccessful = withIso _List (const id) p1
-    , _dmbrFailed     = withIso _List (const id) p2
+deleteMessageBatchResponse :: DeleteMessageBatchResponse
+deleteMessageBatchResponse = DeleteMessageBatchResponse
+    { _dmbrSuccessful = mempty
+    , _dmbrFailed     = mempty
     }
 
 -- | A list of BatchResultErrorEntry items.
diff --git a/gen/Network/AWS/SQS/GetQueueAttributes.hs b/gen/Network/AWS/SQS/GetQueueAttributes.hs
--- a/gen/Network/AWS/SQS/GetQueueAttributes.hs
+++ b/gen/Network/AWS/SQS/GetQueueAttributes.hs
@@ -71,28 +71,27 @@
 import qualified GHC.Exts
 
 data GetQueueAttributes = GetQueueAttributes
-    { _gqaAttributeNames :: List "AttributeName" Text
+    { _gqaAttributeNames :: List "AttributeName" QueueAttributeName
     , _gqaQueueUrl       :: Text
-    } deriving (Eq, Ord, Show)
+    } deriving (Eq, Show)
 
 -- | 'GetQueueAttributes' constructor.
 --
 -- The fields accessible through corresponding lenses are:
 --
--- * 'gqaAttributeNames' @::@ ['Text']
+-- * 'gqaAttributeNames' @::@ ['QueueAttributeName']
 --
 -- * 'gqaQueueUrl' @::@ 'Text'
 --
 getQueueAttributes :: Text -- ^ 'gqaQueueUrl'
-                   -> [Text] -- ^ 'gqaAttributeNames'
                    -> GetQueueAttributes
-getQueueAttributes p1 p2 = GetQueueAttributes
+getQueueAttributes p1 = GetQueueAttributes
     { _gqaQueueUrl       = p1
-    , _gqaAttributeNames = withIso _List (const id) p2
+    , _gqaAttributeNames = mempty
     }
 
 -- | A list of attributes to retrieve information for.
-gqaAttributeNames :: Lens' GetQueueAttributes [Text]
+gqaAttributeNames :: Lens' GetQueueAttributes [QueueAttributeName]
 gqaAttributeNames =
     lens _gqaAttributeNames (\s a -> s { _gqaAttributeNames = a })
         . _List
@@ -102,23 +101,22 @@
 gqaQueueUrl = lens _gqaQueueUrl (\s a -> s { _gqaQueueUrl = a })
 
 newtype GetQueueAttributesResponse = GetQueueAttributesResponse
-    { _gqarAttributes :: EMap "Attribute" "Name" "Value" Text Text
+    { _gqarAttributes :: EMap "Attribute" "Name" "Value" QueueAttributeName Text
     } deriving (Eq, Show, Monoid, Semigroup)
 
 -- | 'GetQueueAttributesResponse' constructor.
 --
 -- The fields accessible through corresponding lenses are:
 --
--- * 'gqarAttributes' @::@ 'HashMap' 'Text' 'Text'
+-- * 'gqarAttributes' @::@ 'HashMap' 'QueueAttributeName' 'Text'
 --
-getQueueAttributesResponse :: HashMap Text Text -- ^ 'gqarAttributes'
-                           -> GetQueueAttributesResponse
-getQueueAttributesResponse p1 = GetQueueAttributesResponse
-    { _gqarAttributes = withIso _EMap (const id) p1
+getQueueAttributesResponse :: GetQueueAttributesResponse
+getQueueAttributesResponse = GetQueueAttributesResponse
+    { _gqarAttributes = mempty
     }
 
 -- | A map of attributes to the respective values.
-gqarAttributes :: Lens' GetQueueAttributesResponse (HashMap Text Text)
+gqarAttributes :: Lens' GetQueueAttributesResponse (HashMap QueueAttributeName Text)
 gqarAttributes = lens _gqarAttributes (\s a -> s { _gqarAttributes = a }) . _EMap
 
 instance ToPath GetQueueAttributes where
diff --git a/gen/Network/AWS/SQS/ListDeadLetterSourceQueues.hs b/gen/Network/AWS/SQS/ListDeadLetterSourceQueues.hs
--- a/gen/Network/AWS/SQS/ListDeadLetterSourceQueues.hs
+++ b/gen/Network/AWS/SQS/ListDeadLetterSourceQueues.hs
@@ -77,10 +77,9 @@
 --
 -- * 'ldlsqrQueueUrls' @::@ ['Text']
 --
-listDeadLetterSourceQueuesResponse :: [Text] -- ^ 'ldlsqrQueueUrls'
-                                   -> ListDeadLetterSourceQueuesResponse
-listDeadLetterSourceQueuesResponse p1 = ListDeadLetterSourceQueuesResponse
-    { _ldlsqrQueueUrls = withIso _List (const id) p1
+listDeadLetterSourceQueuesResponse :: ListDeadLetterSourceQueuesResponse
+listDeadLetterSourceQueuesResponse = ListDeadLetterSourceQueuesResponse
+    { _ldlsqrQueueUrls = mempty
     }
 
 -- | A list of source queue URLs that have the RedrivePolicy queue attribute
diff --git a/gen/Network/AWS/SQS/ListQueues.hs b/gen/Network/AWS/SQS/ListQueues.hs
--- a/gen/Network/AWS/SQS/ListQueues.hs
+++ b/gen/Network/AWS/SQS/ListQueues.hs
@@ -79,10 +79,9 @@
 --
 -- * 'lqrQueueUrls' @::@ ['Text']
 --
-listQueuesResponse :: [Text] -- ^ 'lqrQueueUrls'
-                   -> ListQueuesResponse
-listQueuesResponse p1 = ListQueuesResponse
-    { _lqrQueueUrls = withIso _List (const id) p1
+listQueuesResponse :: ListQueuesResponse
+listQueuesResponse = ListQueuesResponse
+    { _lqrQueueUrls = mempty
     }
 
 -- | A list of queue URLs, up to 1000 entries.
diff --git a/gen/Network/AWS/SQS/ReceiveMessage.hs b/gen/Network/AWS/SQS/ReceiveMessage.hs
--- a/gen/Network/AWS/SQS/ReceiveMessage.hs
+++ b/gen/Network/AWS/SQS/ReceiveMessage.hs
@@ -72,19 +72,19 @@
 import qualified GHC.Exts
 
 data ReceiveMessage = ReceiveMessage
-    { _rmAttributeNames        :: List "AttributeName" Text
+    { _rmAttributeNames        :: List "AttributeName" QueueAttributeName
     , _rmMaxNumberOfMessages   :: Maybe Int
     , _rmMessageAttributeNames :: List "MessageAttributeName" Text
     , _rmQueueUrl              :: Text
     , _rmVisibilityTimeout     :: Maybe Int
     , _rmWaitTimeSeconds       :: Maybe Int
-    } deriving (Eq, Ord, Show)
+    } deriving (Eq, Show)
 
 -- | 'ReceiveMessage' constructor.
 --
 -- The fields accessible through corresponding lenses are:
 --
--- * 'rmAttributeNames' @::@ ['Text']
+-- * 'rmAttributeNames' @::@ ['QueueAttributeName']
 --
 -- * 'rmMaxNumberOfMessages' @::@ 'Maybe' 'Int'
 --
@@ -97,13 +97,11 @@
 -- * 'rmWaitTimeSeconds' @::@ 'Maybe' 'Int'
 --
 receiveMessage :: Text -- ^ 'rmQueueUrl'
-               -> [Text] -- ^ 'rmAttributeNames'
-               -> [Text] -- ^ 'rmMessageAttributeNames'
                -> ReceiveMessage
-receiveMessage p1 p2 p3 = ReceiveMessage
+receiveMessage p1 = ReceiveMessage
     { _rmQueueUrl              = p1
-    , _rmAttributeNames        = withIso _List (const id) p2
-    , _rmMessageAttributeNames = withIso _List (const id) p3
+    , _rmAttributeNames        = mempty
+    , _rmMessageAttributeNames = mempty
     , _rmMaxNumberOfMessages   = Nothing
     , _rmVisibilityTimeout     = Nothing
     , _rmWaitTimeSeconds       = Nothing
@@ -118,7 +116,7 @@
 -- account number (or the IP address, if anonymous access is allowed) of the
 -- sender. SentTimestamp - returns the time when the message was sent (epoch
 -- time in milliseconds).
-rmAttributeNames :: Lens' ReceiveMessage [Text]
+rmAttributeNames :: Lens' ReceiveMessage [QueueAttributeName]
 rmAttributeNames = lens _rmAttributeNames (\s a -> s { _rmAttributeNames = a }) . _List
 
 -- | The maximum number of messages to return. Amazon SQS never returns more
@@ -169,10 +167,9 @@
 --
 -- * 'rmrMessages' @::@ ['Message']
 --
-receiveMessageResponse :: [Message] -- ^ 'rmrMessages'
-                       -> ReceiveMessageResponse
-receiveMessageResponse p1 = ReceiveMessageResponse
-    { _rmrMessages = withIso _List (const id) p1
+receiveMessageResponse :: ReceiveMessageResponse
+receiveMessageResponse = ReceiveMessageResponse
+    { _rmrMessages = mempty
     }
 
 -- | A list of messages.
diff --git a/gen/Network/AWS/SQS/SendMessage.hs b/gen/Network/AWS/SQS/SendMessage.hs
--- a/gen/Network/AWS/SQS/SendMessage.hs
+++ b/gen/Network/AWS/SQS/SendMessage.hs
@@ -80,13 +80,12 @@
 --
 sendMessage :: Text -- ^ 'smQueueUrl'
             -> Text -- ^ 'smMessageBody'
-            -> HashMap Text MessageAttributeValue -- ^ 'smMessageAttributes'
             -> SendMessage
-sendMessage p1 p2 p3 = SendMessage
+sendMessage p1 p2 = SendMessage
     { _smQueueUrl          = p1
     , _smMessageBody       = p2
-    , _smMessageAttributes = withIso _EMap (const id) p3
     , _smDelaySeconds      = Nothing
+    , _smMessageAttributes = mempty
     }
 
 -- | The number of seconds (0 to 900 - 15 minutes) to delay a specific
diff --git a/gen/Network/AWS/SQS/SendMessageBatch.hs b/gen/Network/AWS/SQS/SendMessageBatch.hs
--- a/gen/Network/AWS/SQS/SendMessageBatch.hs
+++ b/gen/Network/AWS/SQS/SendMessageBatch.hs
@@ -75,11 +75,10 @@
 -- * 'smbQueueUrl' @::@ 'Text'
 --
 sendMessageBatch :: Text -- ^ 'smbQueueUrl'
-                 -> [SendMessageBatchRequestEntry] -- ^ 'smbEntries'
                  -> SendMessageBatch
-sendMessageBatch p1 p2 = SendMessageBatch
+sendMessageBatch p1 = SendMessageBatch
     { _smbQueueUrl = p1
-    , _smbEntries  = withIso _List (const id) p2
+    , _smbEntries  = mempty
     }
 
 -- | A list of SendMessageBatchRequestEntry items.
@@ -103,12 +102,10 @@
 --
 -- * 'smbrSuccessful' @::@ ['SendMessageBatchResultEntry']
 --
-sendMessageBatchResponse :: [SendMessageBatchResultEntry] -- ^ 'smbrSuccessful'
-                         -> [BatchResultErrorEntry] -- ^ 'smbrFailed'
-                         -> SendMessageBatchResponse
-sendMessageBatchResponse p1 p2 = SendMessageBatchResponse
-    { _smbrSuccessful = withIso _List (const id) p1
-    , _smbrFailed     = withIso _List (const id) p2
+sendMessageBatchResponse :: SendMessageBatchResponse
+sendMessageBatchResponse = SendMessageBatchResponse
+    { _smbrSuccessful = mempty
+    , _smbrFailed     = mempty
     }
 
 -- | A list of BatchResultErrorEntry items with the error detail about each
diff --git a/gen/Network/AWS/SQS/SetQueueAttributes.hs b/gen/Network/AWS/SQS/SetQueueAttributes.hs
--- a/gen/Network/AWS/SQS/SetQueueAttributes.hs
+++ b/gen/Network/AWS/SQS/SetQueueAttributes.hs
@@ -48,7 +48,7 @@
 import qualified GHC.Exts
 
 data SetQueueAttributes = SetQueueAttributes
-    { _sqaAttributes :: EMap "Attribute" "Name" "Value" Text Text
+    { _sqaAttributes :: EMap "Attribute" "Name" "Value" QueueAttributeName Text
     , _sqaQueueUrl   :: Text
     } deriving (Eq, Show)
 
@@ -56,16 +56,15 @@
 --
 -- The fields accessible through corresponding lenses are:
 --
--- * 'sqaAttributes' @::@ 'HashMap' 'Text' 'Text'
+-- * 'sqaAttributes' @::@ 'HashMap' 'QueueAttributeName' 'Text'
 --
 -- * 'sqaQueueUrl' @::@ 'Text'
 --
 setQueueAttributes :: Text -- ^ 'sqaQueueUrl'
-                   -> HashMap Text Text -- ^ 'sqaAttributes'
                    -> SetQueueAttributes
-setQueueAttributes p1 p2 = SetQueueAttributes
+setQueueAttributes p1 = SetQueueAttributes
     { _sqaQueueUrl   = p1
-    , _sqaAttributes = withIso _EMap (const id) p2
+    , _sqaAttributes = mempty
     }
 
 -- | A map of attributes to set. The following lists the names, descriptions,
@@ -92,7 +91,7 @@
 -- The parameters for dead letter queue functionality of the source queue.
 -- For more information about RedrivePolicy and dead letter queues, see
 -- Using Amazon SQS Dead Letter Queues in the Amazon SQS Developer Guide.
-sqaAttributes :: Lens' SetQueueAttributes (HashMap Text Text)
+sqaAttributes :: Lens' SetQueueAttributes (HashMap QueueAttributeName Text)
 sqaAttributes = lens _sqaAttributes (\s a -> s { _sqaAttributes = a }) . _EMap
 
 -- | The URL of the Amazon SQS queue to take action on.
diff --git a/gen/Network/AWS/SQS/Types.hs b/gen/Network/AWS/SQS/Types.hs
--- a/gen/Network/AWS/SQS/Types.hs
+++ b/gen/Network/AWS/SQS/Types.hs
@@ -175,11 +175,13 @@
         RedrivePolicy                         -> "RedrivePolicy"
         VisibilityTimeout                     -> "VisibilityTimeout"
 
+instance ToByteString QueueAttributeName
+instance ToHeader     QueueAttributeName
+instance ToQuery      QueueAttributeName
+
 instance FromXML QueueAttributeName where
     parseXML = parseXMLText "QueueAttributeName"
 
-instance ToQuery QueueAttributeName where
-    toQuery = toQuery . toText
 
 data DeleteMessageBatchRequestEntry = DeleteMessageBatchRequestEntry
     { _dmbreId            :: Text
@@ -416,7 +418,7 @@
         ]
 
 data Message = Message
-    { _mAttributes             :: EMap "Attribute" "Name" "Value" Text Text
+    { _mAttributes             :: EMap "Attribute" "Name" "Value" QueueAttributeName Text
     , _mBody                   :: Maybe Text
     , _mMD5OfBody              :: Maybe Text
     , _mMD5OfMessageAttributes :: Maybe Text
@@ -429,7 +431,7 @@
 --
 -- The fields accessible through corresponding lenses are:
 --
--- * 'mAttributes' @::@ 'HashMap' 'Text' 'Text'
+-- * 'mAttributes' @::@ 'HashMap' 'QueueAttributeName' 'Text'
 --
 -- * 'mBody' @::@ 'Maybe' 'Text'
 --
@@ -443,24 +445,22 @@
 --
 -- * 'mReceiptHandle' @::@ 'Maybe' 'Text'
 --
-message :: HashMap Text Text -- ^ 'mAttributes'
-        -> HashMap Text MessageAttributeValue -- ^ 'mMessageAttributes'
-        -> Message
-message p1 p2 = Message
-    { _mAttributes             = withIso _EMap (const id) p1
-    , _mMessageAttributes      = withIso _EMap (const id) p2
-    , _mMessageId              = Nothing
+message :: Message
+message = Message
+    { _mMessageId              = Nothing
     , _mReceiptHandle          = Nothing
     , _mMD5OfBody              = Nothing
     , _mBody                   = Nothing
+    , _mAttributes             = mempty
     , _mMD5OfMessageAttributes = Nothing
+    , _mMessageAttributes      = mempty
     }
 
 -- | SenderId, SentTimestamp, ApproximateReceiveCount, and/or
 -- ApproximateFirstReceiveTimestamp. SentTimestamp and
 -- ApproximateFirstReceiveTimestamp are each returned as an integer
 -- representing the epoch time in milliseconds.
-mAttributes :: Lens' Message (HashMap Text Text)
+mAttributes :: Lens' Message (HashMap QueueAttributeName Text)
 mAttributes = lens _mAttributes (\s a -> s { _mAttributes = a }) . _EMap
 
 -- | The message's contents (not URL-encoded).
@@ -540,13 +540,12 @@
 --
 sendMessageBatchRequestEntry :: Text -- ^ 'smbreId'
                              -> Text -- ^ 'smbreMessageBody'
-                             -> HashMap Text MessageAttributeValue -- ^ 'smbreMessageAttributes'
                              -> SendMessageBatchRequestEntry
-sendMessageBatchRequestEntry p1 p2 p3 = SendMessageBatchRequestEntry
+sendMessageBatchRequestEntry p1 p2 = SendMessageBatchRequestEntry
     { _smbreId                = p1
     , _smbreMessageBody       = p2
-    , _smbreMessageAttributes = withIso _EMap (const id) p3
     , _smbreDelaySeconds      = Nothing
+    , _smbreMessageAttributes = mempty
     }
 
 -- | The number of seconds for which the message has to be delayed.
