Symfony Swiftmailer multiple mailers with fallback

App_Swift_Transport_FailoverTransport .php

<?php

namespace AppBundle\Util;

class App_Swift_Transport_FailoverTransport extends \Swift_Transport_FailoverTransport
{
    /**
     * @param \Swift_Mailer[] $mailers
     */
    public function __construct(array $mailers)
    {
        parent::__construct();

        /** @var \Swift_Mailer $mailer */
        foreach ($mailers as $mailer) {
            $this->_transports[] = $mailer->getTransport();
        }
    }
}

config.yml

swiftmailer:
    default_mailer:       default
    mailers:
        default:
            transport: 'app_failover'
        # example mailhog config 
        main:   
            disable_delivery: false
            delivery_addresses: null
            transport: smtp
            host: 127.0.0.1
            port: 1025
            username: null
            password: null
        # example for sendgrid 
        fallback:
            disable_delivery: false
            delivery_addresses: null
            encryption: 'ssl'
            transport: 'smtp'
            host: 'smtp.sendgrid.net'
            port: '465'
            username: 'apikey'
            password: 'mypassword'

service.yml

swiftmailer.mailer.transport.app_failover:
    class: AppBundle\Util\App_Swift_Transport_FailoverTransport
    arguments:
        $mailers: ['@swiftmailer.mailer.main', '@swiftmailer.mailer.fallback']

See other transports: https://swiftmailer.symfony.com/docs/sending.html#transport-types