diff --git a/README.md b/README.md
index 8777423a..c1acf4e1 100644
--- a/README.md
+++ b/README.md
@@ -104,8 +104,12 @@ func main() {
 
 * Seemingly random disconnections may be caused by another client connecting to the broker with the same client 
 identifier; this is as per the [spec](https://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc384800405).
-* A `MessageHandler` (called when a new message is received) must not block. If you wish to perform a long-running task,
-or publish a message, then please use a go routine (blocking in the handler is a common cause of unexpected `pingresp 
+* Unless ordered delivery of messages is essential (and you have configured your broker to support this e.g. 
+  `max_inflight_messages=1` in mosquitto) then set `ClientOptions.SetOrderMatters(false)`. Doing so will avoid the 
+  below issue (deadlocks due to blocking message handlers).
+* A `MessageHandler` (called when a new message is received) must not block (unless 
+  `ClientOptions.SetOrderMatters(false)` set). If you wish to perform a long-running task, or publish a message, then 
+  please use a go routine (blocking in the handler is a common cause of unexpected `pingresp 
 not received, disconnecting` errors). 
 * When QOS1+ subscriptions have been created previously and you connect with `CleanSession` set to false it is possible that the broker will deliver retained 
 messages before `Subscribe` can be called. To process these messages either configure a handler with `AddRoute` or
diff --git a/client.go b/client.go
index 5b4b2e7f..8558f3bf 100644
--- a/client.go
+++ b/client.go
@@ -55,6 +55,8 @@ const (
 // information can be found in their respective documentation.
 // Numerous connection options may be specified by configuring a
 // and then supplying a ClientOptions type.
+// Implementations of Client must be safe for concurrent use by multiple
+// goroutines
 type Client interface {
 	// IsConnected returns a bool signifying whether
 	// the client is connected or not.
@@ -75,11 +77,21 @@ type Client interface {
 	// Returns a token to track delivery of the message to the broker
 	Publish(topic string, qos byte, retained bool, payload interface{}) Token
 	// Subscribe starts a new subscription. Provide a MessageHandler to be executed when
-	// a message is published on the topic provided, or nil for the default handler
+	// a message is published on the topic provided, or nil for the default handler.
+	//
+	// If `options.OrderMatters` is true (the default) then `callback` must not block or
+	// call functions within this package that may block (e.g. `Publish`) other than in
+	// a new go routine.
+	// `callback` must be safe for concurrent use by multiple goroutines.
 	Subscribe(topic string, qos byte, callback MessageHandler) Token
 	// SubscribeMultiple starts a new subscription for multiple topics. Provide a MessageHandler to
 	// be executed when a message is published on one of the topics provided, or nil for the
-	// default handler
+	// default handler.
+	//
+	// If `options.OrderMatters` is true (the default) then `callback` must not block or
+	// call functions within this package that may block (e.g. `Publish`) other than in
+	// a new go routine.
+	// `callback` must be safe for concurrent use by multiple goroutines.
 	SubscribeMultiple(filters map[string]byte, callback MessageHandler) Token
 	// Unsubscribe will end the subscription from each of the topics provided.
 	// Messages published to those topics from other clients will no longer be
@@ -87,7 +99,13 @@ type Client interface {
 	Unsubscribe(topics ...string) Token
 	// AddRoute allows you to add a handler for messages on a specific topic
 	// without making a subscription. For example having a different handler
-	// for parts of a wildcard subscription
+	// for parts of a wildcard subscription or for receiving retained messages
+	// upon connection (before Sub scribe can be processed).
+	//
+	// If `options.OrderMatters` is true (the default) then `callback` must not block or
+	// call functions within this package that may block (e.g. `Publish`) other than in
+	// a new go routine.
+	// `callback` must be safe for concurrent use by multiple goroutines.
 	AddRoute(topic string, callback MessageHandler)
 	// OptionsReader returns a ClientOptionsReader which is a copy of the clientoptions
 	// in use by the client.
@@ -95,6 +113,8 @@ type Client interface {
 }
 
 // client implements the Client interface
+// clients are safe for concurrent use by multiple
+// goroutines
 type client struct {
 	lastSent        atomic.Value // time.Time - the last time a packet was successfully sent to network
 	lastReceived    atomic.Value // time.Time - the last time a packet was successfully received from network
@@ -153,6 +173,11 @@ func NewClient(o *ClientOptions) Client {
 // AddRoute allows you to add a handler for messages on a specific topic
 // without making a subscription. For example having a different handler
 // for parts of a wildcard subscription
+//
+// If `options.OrderMatters` is true (the default) then `callback` must not block or
+// call functions within this package that may block (e.g. `Publish`) other than in
+// a new go routine.
+// `callback` must be safe for concurrent use by multiple goroutines.
 func (c *client) AddRoute(topic string, callback MessageHandler) {
 	if callback != nil {
 		c.msgRouter.addRoute(topic, callback)
@@ -686,10 +711,10 @@ func (c *client) Publish(topic string, qos byte, retained bool, payload interfac
 // Subscribe starts a new subscription. Provide a MessageHandler to be executed when
 // a message is published on the topic provided.
 //
-// Please note: you should try to keep the execution time of the callback to be
-// as low as possible, especially when SetOrderMatters(true) (the default) is in
-// place. Blocking calls in message handlers might otherwise delay delivery to
-// other message handlers.
+// If `options.OrderMatters` is true (the default) then `callback` must not block or
+// call functions within this package that may block (e.g. `Publish`) other than in
+// a new go routine.
+// `callback` must be safe for concurrent use by multiple goroutines.
 func (c *client) Subscribe(topic string, qos byte, callback MessageHandler) Token {
 	token := newToken(packets.Subscribe).(*SubscribeToken)
 	DEBUG.Println(CLI, "enter Subscribe")
@@ -766,6 +791,11 @@ func (c *client) Subscribe(topic string, qos byte, callback MessageHandler) Toke
 
 // SubscribeMultiple starts a new subscription for multiple topics. Provide a MessageHandler to
 // be executed when a message is published on one of the topics provided.
+//
+// If `options.OrderMatters` is true (the default) then `callback` must not block or
+// call functions within this package that may block (e.g. `Publish`) other than in
+// a new go routine.
+// `callback` must be safe for concurrent use by multiple goroutines.
 func (c *client) SubscribeMultiple(filters map[string]byte, callback MessageHandler) Token {
 	var err error
 	token := newToken(packets.Subscribe).(*SubscribeToken)
diff --git a/cmd/docker/binds/mosquitto/config/mosquitto.conf b/cmd/docker/binds/mosquitto/config/mosquitto.conf
index da13df71..6a50fd76 100644
--- a/cmd/docker/binds/mosquitto/config/mosquitto.conf
+++ b/cmd/docker/binds/mosquitto/config/mosquitto.conf
@@ -7,8 +7,9 @@
 
 allow_anonymous true # Anyone can connect
 
-# Port to use for the default listener.
-#port 1883
+# Mosquitto v2+ requires that a listener be definer (otherwise it listens on loopback)
+listener 1883
+
 
 #log_type error
 #log_type warning
diff --git a/cmd/docker/publisher/main.go b/cmd/docker/publisher/main.go
index b98d3bf7..1e6b3152 100644
--- a/cmd/docker/publisher/main.go
+++ b/cmd/docker/publisher/main.go
@@ -20,6 +20,8 @@ const (
 	SERVERADDRESS = "tcp://mosquitto:1883"
 	DELAY         = time.Second
 	CLIENTID      = "mqtt_publisher"
+
+	WRITETOLOG = true // If true then published messages will be written to the console
 )
 
 func main() {
@@ -32,6 +34,7 @@ func main() {
 	opts.AddBroker(SERVERADDRESS)
 	opts.SetClientID(CLIENTID)
 
+	opts.SetOrderMatters(false)       // Allow out of order messages (use this option unless in order delivery is essential)
 	opts.ConnectTimeout = time.Second // Minimal delays on connect
 	opts.WriteTimeout = time.Second   // Minimal delays on writes
 	opts.KeepAlive = 10               // Keepalive every 10 seconds so we quickly detect network outages
@@ -85,6 +88,9 @@ func main() {
 					panic(err)
 				}
 
+				if WRITETOLOG {
+					fmt.Printf("sending message: %s\n", msg)
+				}
 				t := client.Publish(TOPIC, QOS, false, msg)
 				// Handle the token in a go routine so this loop keeps sending messages regardless of delivery status
 				go func() {
diff --git a/cmd/docker/subscriber/main.go b/cmd/docker/subscriber/main.go
index fc0148c6..fffca909 100644
--- a/cmd/docker/subscriber/main.go
+++ b/cmd/docker/subscriber/main.go
@@ -93,6 +93,7 @@ func main() {
 	opts.AddBroker(SERVERADDRESS)
 	opts.SetClientID(CLIENTID)
 
+	opts.SetOrderMatters(false)       // Allow out of order messages (use this option unless in order delivery is essential)
 	opts.ConnectTimeout = time.Second // Minimal delays on connect
 	opts.WriteTimeout = time.Second   // Minimal delays on writes
 	opts.KeepAlive = 10               // Keepalive every 10 seconds so we quickly detect network outages
diff --git a/fvt/docker/runTests.cmd b/fvt/docker/runTests.cmd
index 9a738613..7d38e65e 100644
--- a/fvt/docker/runTests.cmd
+++ b/fvt/docker/runTests.cmd
@@ -1,9 +1,22 @@
 @ECHO OFF
 REM Windows CMD file to run golang Paho tests with docker mosquitto instance
 cls
+:start
 docker-compose up -d
 REM Docker for windows does not support publishing to 127.0.0.1 so set the address for the tests to use.
 set TEST_FVT_ADDR=0.0.0.0
-go test -v ../../
+REM `--count 1` prevents the system from using cached results. Note that running the tests multiple times may fail
+REM because the broker state will not be as expected
+go test --count 1 -v ../../
 rem go test -race -v ../../
-docker-compose down
\ No newline at end of file
+IF ERRORLEVEL   1 GOTO failed
+GOTO successful
+
+:failed
+docker-compose down
+echo "Error"
+exit
+
+:successful
+docker-compose down
+REM goto start
\ No newline at end of file
diff --git a/options.go b/options.go
index eb475918..ee2e71b0 100644
--- a/options.go
+++ b/options.go
@@ -49,7 +49,8 @@ type OnConnectHandler func(Client)
 // the initial connection is lost
 type ReconnectHandler func(Client, *ClientOptions)
 
-// ClientOptions contains configurable options for an Client.
+// ClientOptions contains configurable options for an Client. Note that these should be set using the
+// relevant methods (e.g. `AddBroker`) rather than directly. See those functions for information on usage.
 type ClientOptions struct {
 	Servers                 []*url.URL
 	ClientID                string
@@ -89,7 +90,7 @@ type ClientOptions struct {
 // default values.
 //   Port: 1883
 //   CleanSession: True
-//   Order: True
+//   Order: True (note: it is recommended that this be set to FALSE unless order is important)
 //   KeepAlive: 30 (seconds)
 //   ConnectTimeout: 30 (seconds)
 //   MaxReconnectInterval 10 (minutes)
@@ -203,10 +204,13 @@ func (o *ClientOptions) SetCleanSession(clean bool) *ClientOptions {
 }
 
 // SetOrderMatters will set the message routing to guarantee order within
-// each QoS level. By default, this value is true. If set to false,
+// each QoS level. By default, this value is true. If set to false (recommended),
 // this flag indicates that messages can be delivered asynchronously
 // from the client to the application and possibly arrive out of order.
 // Specifically, the message handler is called in its own go routine.
+// Note that setting this to true does not guarantee in-order delivery
+// (this is subject to broker settings like `max_inflight_messages=1` in mosquitto)
+// and if `true` then handlers must not block.
 func (o *ClientOptions) SetOrderMatters(order bool) *ClientOptions {
 	o.Order = order
 	return o
@@ -286,6 +290,11 @@ func (o *ClientOptions) SetBinaryWill(topic string, payload []byte, qos byte, re
 
 // SetDefaultPublishHandler sets the MessageHandler that will be called when a message
 // is received that does not match any known subscriptions.
+//
+// If `options.OrderMatters` is true (the default) then `defaultHandler` must not block or
+// call functions within this package that may block (e.g. `Publish`) other than in
+// a new go routine.
+// `defaultHandler` must be safe for concurrent use by multiple goroutines.
 func (o *ClientOptions) SetDefaultPublishHandler(defaultHandler MessageHandler) *ClientOptions {
 	o.DefaultPublishHandler = defaultHandler
 	return o