diff --git a/.travis.yml b/.travis.yml
index 9a0646f..5820f49 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,5 +1,6 @@
 language: php
 php:
+    - 5.6
     - 7.0
     - 7.1
     - 7.2
@@ -7,10 +8,19 @@ php:
     - hhvm
 
 env:
-    - DEPENDENCIES="high"
-    - DEPENDENCIES="low"
+    - SWIFTMAILER=^4.0 DEPENDENCIES=high
+    - SWIFTMAILER=^4.0 DEPENDENCIES=low
+    - SWIFTMAILER=^5.0 DEPENDENCIES=high
+    - SWIFTMAILER=^5.0 DEPENDENCIES=low
+    - SWIFTMAILER=^6.0 DEPENDENCIES=high
+    - SWIFTMAILER=^6.0 DEPENDENCIES=low
 
 matrix:
+    exclude:
+        - php: 5.6
+          env: SWIFTMAILER=^6.0 DEPENDENCIES=high
+        - php: 5.6
+          env: SWIFTMAILER=^6.0 DEPENDENCIES=low
     allow_failures:
         - php: hhvm
         - php: nightly
@@ -20,9 +30,11 @@ cache:
         - $HOME/.composer/cache
 
 install:
-    - if [[ "$DEPENDENCIES" = 'high' ]]; then composer update; fi
-    - if [[ "$DEPENDENCIES" = 'low' ]]; then composer update --prefer-lowest; fi
-    - composer require php-coveralls/php-coveralls '^2.0' --dev
+    - if [[ ! -z $SWIFTMAILER ]]; then composer require "swiftmailer/swiftmailer $SWIFTMAILER" --no-update; fi
+    - if [[ ${TRAVIS_PHP_VERSION:0:3} > 7.1 ]]; then composer require "phpunit/phpunit >=6.0 <8" --dev --no-update; fi # PHPUnit 5 is installed in PHP 7.2 despite it doesn't work in PHP 7.2
+    - if [[ $DEPENDENCIES = high ]]; then composer update; fi
+    - if [[ $DEPENDENCIES = low ]]; then composer update --prefer-lowest; fi
+    - composer require php-coveralls/php-coveralls ^2.0 --dev
 
 script:
     - php vendor/bin/phpunit --coverage-clover ./tests/logs/clover.xml
diff --git a/composer.json b/composer.json
index e422e6e..13c23ff 100644
--- a/composer.json
+++ b/composer.json
@@ -12,11 +12,11 @@
 		}
 	],
 	"require": {
-		"php": ">=7.0",
-		"swiftmailer/swiftmailer": "^6.0"
+		"php": ">=5.6",
+		"swiftmailer/swiftmailer": ">=4.0 <7.0"
 	},
 	"require-dev": {
-		"phpunit/phpunit": "^6.4",
+		"phpunit/phpunit": ">=5.4.3 <8.0",
 		"mockery/mockery": "^1.0"
 	},
 	"autoload": {
diff --git a/src/SwiftMailerDefaultsPlugin.php b/src/SwiftMailerDefaultsPlugin.php
index b345bdb..e3ef8ad 100644
--- a/src/SwiftMailerDefaultsPlugin.php
+++ b/src/SwiftMailerDefaultsPlugin.php
@@ -17,10 +17,10 @@ class SwiftMailerDefaultsPlugin implements \Swift_Events_EventListener, \Swift_E
     protected $defaults = [];
 
     /**
-     * @var \Swift_Mime_SimpleMessage|null Message that was received before sending and is not modified. It is kept to
-     *     restore the original message after sending.
+     * @var array The original values of a sent message properties which were replaced by the default properties. They
+     *     are kept to restore the original message properties after the sending.
      */
-    protected $originalMessage;
+    protected $originalValues = [];
 
     /**
      * @param array $defaults Default Message properties. See the readme for more information.
@@ -40,7 +40,7 @@ public function __construct(array $defaults = [])
      * @param string $property The property name. See the readme for more information.
      * @param array ...$arguments The list of argument for the Swift_Mime_SimpleMessage property setter
      */
-    public function setDefault(string $property, ...$arguments)
+    public function setDefault($property, ...$arguments)
     {
         $this->defaults[ucfirst($property)] = $arguments;
     }
@@ -50,7 +50,7 @@ public function setDefault(string $property, ...$arguments)
      *
      * @param string $property The property name. See the readme for more information.
      */
-    public function unsetDefault(string $property)
+    public function unsetDefault($property)
     {
         unset($this->defaults[ucfirst($property)]);
     }
@@ -61,11 +61,11 @@ public function unsetDefault(string $property)
     public function beforeSendPerformed(\Swift_Events_SendEvent $event)
     {
         $message = $event->getMessage();
-        $this->originalMessage = clone $message;
 
         foreach ($this->defaults as $property => $arguments) {
             $originalValue = $message->{'get'.$property}();
             if ($originalValue === null || $originalValue === '' || $originalValue === []) {
+                $this->originalValues[$property] = $originalValue;
                 $message->{'set'.$property}(...$arguments);
             }
         }
@@ -78,11 +78,10 @@ public function sendPerformed(\Swift_Events_SendEvent $event)
     {
         $message = $event->getMessage();
 
-        foreach ($this->defaults as $property => $arguments) {
-            $originalValue = $this->originalMessage->{'get'.$property}();
+        foreach ($this->originalValues as $property => $originalValue) {
             $message->{'set'.$property}($originalValue);
         }
 
-        $this->originalMessage = null;
+        $this->originalValues = [];
     }
 }
diff --git a/tests/SwiftMailerDefaultsPluginTest.php b/tests/SwiftMailerDefaultsPluginTest.php
index 9c5add5..1932fb0 100644
--- a/tests/SwiftMailerDefaultsPluginTest.php
+++ b/tests/SwiftMailerDefaultsPluginTest.php
@@ -12,6 +12,19 @@
  */
 class SwiftMailerDefaultsPluginTest extends TestCase
 {
+    /**
+     * @inheritDoc
+     */
+    public static function setUpBeforeClass()
+    {
+        parent::setUpBeforeClass();
+
+        // Early SwiftMailer 4 versions don't support Composer autoload
+        if (!class_exists('\Swift_Message')) {
+            require __DIR__.'/../vendor/swiftmailer/swiftmailer/lib/swift_required.php';
+        }
+    }
+
     /**
      * Tests passing default values to Message
      */