I'm using the Symfony mailer in a custom class in a Symfony 6 project. I'm using autowiring via type hints in the class's constructor like this:

class MyClass {
        public function __construct(private readonly MailerInterface $mailer) {}


        public function sendEmail(): array
        {
            // Email is sent down here
            try {
                $this->mailer->send($email);
            
                return [
                    'success' => true,
                    'message' => 'Email sent',
                ];
            } catch (TransportExceptionInterface $e) {
                return [
                    'success' => false,
                    'message' => 'Error sending email: ' . $e,
                ];
            }
        }
    }

Call the sendEmail() method in the controller and everything is fine.

Now I want to test whether TransportExceptions are handled correctly. To do this, I need the mailer to throw TransportExceptions in my tests. However, this didn't work as I hoped.

Note: I cannot throw an exception by passing an invalid email address because the sendMail method only allows valid email addresses.

Things I've tried:

1) Use simulated email program

// boot kernel and get Class from container
$container = self::getContainer();
$myClass = $container->get('AppModelMyClass');

// create mock mailer service
$mailer = $this->createMock(Mailer::class);
$mailer->method('send')
        ->willThrowException(new TransportException());
$container->set('SymfonyComponentMailerMailer', $mailer);

As a result I cannot mock the Mailer class because it is final.

2) Use a mock (or stub) MailerInterface

// create mock mailer service
$mailer = $this->createStub(MailerInterface::class);
$mailer->method('send')
        ->willThrowException(new TransportException());
$container->set('SymfonyComponentMailerMailer', $mailer);

No errors, but no exceptions are thrown. The mail service does not appear to have been replaced.

3) Use a custom MailerExceptionTester class

// MailerExceptionTester.php
<?php

namespace AppTests;

use SymfonyComponentMailerEnvelope;
use SymfonyComponentMailerExceptionTransportException;
use SymfonyComponentMailerMailerInterface;
use SymfonyComponentMimeRawMessage;

/**
 * Always throws a TransportException
 */
final class MailerExceptionTester implements MailerInterface
{
    public function send(RawMessage $message, Envelope $envelope = null): void
    {
        throw new TransportException();
    }
}

In testing:

// create mock mailer service
$mailer = new MailerExceptionTester();
$container->set('SymfonyComponentMailerMailer', $mailer);

Same result as in 2)

4) Try changing the MailerInterface service instead of Mailer

// create mock mailer service
$mailer = $this->createMock(MailerInterface::class);
$mailer->method('send')
        ->willThrowException(new TransportException());
$container->set('SymfonyComponentMailerMailerInterface', $mailer);

Error message: SymfonyComponentDependencyInjectionExceptionInvalidArgumentException: The 'SymfonyComponentMailerMailerInterface' service is private and you cannot replace it.

5) Set MailerInterface to public

// services.yaml
services:
    SymfonyComponentMailerMailerInterface:
        public: true

Error: Unable to instantiate interface SymfonyComponentMailerMailerInterface

6) Add an alias for MailerInterface

// services.yaml
services:
    app.mailer:
        alias: SymfonyComponentMailerMailerInterface
        public: true

Error message: SymfonyComponentDependencyInjectionExceptionInvalidArgumentException: The 'SymfonyComponentMailerMailerInterface' service is private and you cannot replace it.

How to replace the auto-connected MailerInterface service in a test?