diff --git a/src/TestSessionEnvironment.php b/src/TestSessionEnvironment.php
index 0a729e3..4d63233 100644
--- a/src/TestSessionEnvironment.php
+++ b/src/TestSessionEnvironment.php
@@ -21,6 +21,7 @@ use SilverStripe\ORM\DatabaseAdmin;
 use SilverStripe\ORM\DB;
 use SilverStripe\ORM\FieldType\DBDatetime;
 use SilverStripe\Versioned\Versioned;
+use Symfony\Component\Mailer\MailerInterface;
 use stdClass;
 
 /**
@@ -320,15 +321,13 @@ class TestSessionEnvironment
         }
 
         // Mailer
-        $mailer = (isset($state->mailer)) ? $state->mailer : null;
-
-        if ($mailer) {
-            if (!class_exists($mailer ?? '') || !is_subclass_of($mailer, 'SilverStripe\\Control\\Email\\Mailer')) {
-                throw new InvalidArgumentException(sprintf(
-                    'Class "%s" is not a valid class, or subclass of Mailer',
-                    $mailer
-                ));
-            }
+        $mailer = $state->mailer ?? null;
+        if ($mailer && !is_a($mailer, MailerInterface::class, true)) {
+            throw new InvalidArgumentException(sprintf(
+                'Class "%s" does not implement %s',
+                $mailer,
+                MailerInterface::class
+            ));
         }
 
         // Date and time
diff --git a/src/TestSessionHTTPMiddleware.php b/src/TestSessionHTTPMiddleware.php
index e3d20a8..26ae43a 100644
--- a/src/TestSessionHTTPMiddleware.php
+++ b/src/TestSessionHTTPMiddleware.php
@@ -4,12 +4,14 @@ namespace SilverStripe\TestSession;
 
 use SilverStripe\Control\Director;
 use SilverStripe\Control\Email\Email;
-use SilverStripe\Control\Email\Mailer;
 use SilverStripe\Control\HTTPRequest;
 use SilverStripe\Control\Middleware\HTTPMiddleware;
 use SilverStripe\Core\Injector\Injector;
 use SilverStripe\ORM\DB;
 use SilverStripe\ORM\FieldType\DBDatetime;
+use Symfony\Component\EventDispatcher\EventDispatcherInterface;
+use Symfony\Component\Mailer\MailerInterface;
+use Symfony\Component\Mailer\Transport\NullTransport;
 
 /**
  * Sets state previously initialized through {@link TestSessionController}.
@@ -67,7 +69,12 @@ class TestSessionHTTPMiddleware implements HTTPMiddleware
         // Register mailer
         if (isset($testState->mailer)) {
             $mailer = $testState->mailer;
-            Injector::inst()->registerService(new $mailer(), Mailer::class);
+            $dispatcher = Injector::inst()->get(EventDispatcherInterface::class . '.mailer');
+            $transport = new NullTransport($dispatcher);
+            Injector::inst()->registerService(
+                new $mailer($transport, $dispatcher),
+                MailerInterface::class
+            );
             Email::config()->set("send_all_emails_to", null);
             Email::config()->update('admin_email', 'no-reply@example.com');
         }