how to test if mailable fails in laravel?

Question

I am testing my laravel app, I am checking it with code coverage, so, everything is working fine except this line:

<?php

namespace App\Mail;

use App\Models\Post;
use Illuminate\Bus\Queueable;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Mail\Mailable;
use Illuminate\Queue\SerializesModels;
use Illuminate\Support\Facades\Log;
use Illuminate\Support\Facades\URL;
use Throwable;

class PostCreated extends Mailable
{
    use Queueable, SerializesModels;

    protected $post;

    /**
     * Create a new message instance.
     *
     * @return void
     */
    public function __construct(Post $post)
    {
        $this->post = $post;  
    }    

    /**
     * Build the message.
     *
     * @return $this
     */
    public function build()
    {                                    

        return $this->subject('Subject')
            ->from(config('mail.from.address'))
            ->to($this->post->user->email)
            ->markdown('emails.posts.created', $this->post);
    }  
    
    /**
     * Handle a job failure.
     *
     * @param  \Throwable  $exception
     * @return void
     */
    public function failed(Throwable $exception)
    {
        //I will add this to a database logs later, but for the moment I just want to log it
        Log::error('Send email to this user was no possible '.
            $this->post->user->email
        );        
    }
}

The code coverage report shows the failed method was not tested, I am trying to test it but I can't, I suppose that I need to make the PostCreated mailable fails in my test, but I don't know how. thanks.

EDIT 1:

PostTest.php

public function test_a_it_throws_if_can_not_send_email()
    {                            

        $this->expectException(\Throwable::class);

        $listing = Post::factory()->create(); 
        
        $mailable = new PostCreated($post);
        $message = $mailable->failed(new Throwable('error sending email'));

    } 

This test pass, but when the code coverage reports still shows that it needs to be tested. So I modified the test and the code:

PostCreated.php

...
   ...
    public function failed(Throwable $exception)
    {
        
    Log::error('Send email to this user was no possible '.
        $this->post->user->email
    ); 
    //new 
    return $exception->getMessage();
    }

PostTest.php

public function test_a_it_throws_if_can_not_send_email()
{ 
  $listing = Post::factory()->create();         
  $mailable = new PostCreated($listing, 300);
        
  $message = $mailable->failed(new \Exception('error sending email'));

  $this->assertEquals($message, 'error sending email');
}

This test passes and code coverage reports is ok now, I don't know if this is the best approach but it works, maybe if you have something better let me know. thanks.