SwiftMailer with file spool doesn't store emails - symfony

Question

I try to set up SwiftMailer to use the spool file system. I'd follow the steps in the Symfony 3.4 documentation and I can't make it work like I want, the emails keep sending and there are not stocked in the folder I want.
There is my configuration :
config.yml
swiftmailer:
transport: '%mailer_transport%'
host: '%mailer_host%'
username: '%mailer_user%'
password: '%mailer_password%'
spool:
type: file
path: '%kernel.project_dir%/src/to/folder/Spool'
disable_delivery: true
PHP
$transport = new \Swift_SmtpTransport();
$mailer = new \Swift_Mailer($transport);
$message = (new \Swift_Message())
->setSubject('Test of spool email')
->setFrom(['test#mail.com' => 'Test mailer'])
->setTo('fake#mail.com')
->setBody("Email de test");
foreach($listeDestinataires as $desitnataire) {
$mailer->send($message);
}
I tried to empty dev and prod cache, but with no effects.
If I do php bin/console swiftmailer:debug I have this result :
Configured SwiftMailer Mailers
==============================
-------------------------- ----------- ------- ---------- ----------------
Name Transport Spool Delivery Single Address
-------------------------- ----------- ------- ---------- ----------------
default (default mailer) null YES NO
-------------------------- ----------- ------- ---------- ----------------
I don't know what I'm doing wrong...

Answer 1

Are you sending in the correct enviroment DEV or PROD?
Also try this: path: '%kernel.project_dir%/var/spool'
Does the directory exists?
Ex.: https://symfony.com/doc/current/email/spool.html