--- a/Transport/MicrosoftGraphTransportFactory.php
+++ b/Transport/MicrosoftGraphTransportFactory.php
@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Mailer\Bridge\MicrosoftGraph\Transport;
 
+use Symfony\Component\HttpClient\HttpClient;
 use Symfony\Component\Mailer\Bridge\MicrosoftGraph\TokenManager;
 use Symfony\Component\Mailer\Exception\IncompleteDsnException;
 use Symfony\Component\Mailer\Exception\InvalidArgumentException;
@@ -52,9 +53,12 @@
             throw new InvalidArgumentException('Graph endpoint needs to be provided without "http(s)://".');
         }
 
-        $tokenManager = new TokenManager($graphEndpoint, $authEndpoint, $tenantId, $this->getUser($dsn), $this->getPassword($dsn), $this->client);
+        // Transport::getDefaultFactories() creates this factory without an HTTP
+        // client, which TokenManager requires.
+        $client = $this->client ?? HttpClient::create();
+        $tokenManager = new TokenManager($graphEndpoint, $authEndpoint, $tenantId, $this->getUser($dsn), $this->getPassword($dsn), $client);
 
-        return new MicrosoftGraphApiTransport($graphEndpoint, $tokenManager, $dsn->getBooleanOption('noSave'), $this->client, $this->dispatcher, $this->logger);
+        return new MicrosoftGraphApiTransport($graphEndpoint, $tokenManager, $dsn->getBooleanOption('noSave'), $client, $this->dispatcher, $this->logger);
     }
 
     protected function getSupportedSchemes(): array