[Mailer] Add a more precise exception

Author: fabpot

Tests/Transport/TransportsTest.php

@@ -12,6 +12,7 @@
 namespace Symfony\Component\Mailer\Tests\Transport;
 
 use PHPUnit\Framework\TestCase;
+use Symfony\Component\Mailer\Exception\InvalidArgumentException;
 use Symfony\Component\Mailer\Transport\TransportInterface;
 use Symfony\Component\Mailer\Transport\Transports;
 use Symfony\Component\Mime\Header\Headers;
@@ -48,4 +49,19 @@ public function testOverrideTransport()
         $email = new Message($headers, new TextPart('...'));
         $transport->send($email);
     }
+
+    public function testTransportDoesNotExist()
+    {
+        $transport = new Transports([
+            'foo' => $this->createMock(TransportInterface::class),
+            'bar' => $this->createMock(TransportInterface::class),
+        ]);
+
+        $headers = (new Headers())->addTextHeader('X-Transport', 'foobar');
+        $email = new Message($headers, new TextPart('...'));
+
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('The "foobar" transport does not exist (available transports: "foo", "bar").');
+        $transport->send($email);
+    }
 }

Transport/Transports.php

@@ -56,7 +56,7 @@ public function send(RawMessage $message, SmtpEnvelope $envelope = null): ?SentM
         $headers->remove('X-Transport');
 
         if (!isset($this->transports[$transport])) {
-            throw new InvalidArgumentException(sprintf('The "%s" transport does not exist.', $transport));
+            throw new InvalidArgumentException(sprintf('The "%s" transport does not exist (available transports: "%s").', $transport, implode('", "', array_keys($this->transports))));
         }
 
         return $this->transports[$transport]->send($message, $envelope);