Symfony2: using Swift_FailoverTransport with Swift_SpoolTransport

services.yml

services:
    # A spooling implementation.
    acme.spool.queued:
        class: Acme\Mailer\QueuedSpool

    # The transport using the spool above.
    acme.transport.spool:
        class: Swift_SpoolTransport
        arguments:
            - '@acme.spool.queued'

    # The transport to be used.
    acme.failover_transport:
        class: "%swiftmailer.transport.failover.class%"
        calls:
            # This call sets the transports:
            # The first one will be the configured transport (like smtp, ..) utilizing SwiftmailerBundle configuration.
            # The second transport in use is the spool transport defined above.
            - [ 'setTransports', [ [ '@swiftmailer.transport', '@acme.transport.spool' ] ] ]

    # This alias is required to have the "swiftmailer:spool:send" command working.
    # The current stable release is not capable of utilizing it, see https://github.com/symfony/SwiftmailerBundle/pull/40
    swiftmailer.transport.real:
        alias: acme.failover_transport

    # Overwriting the configured mailer service to use the failover instead of the configured transport.
    swiftmailer.mailer:
        class: '%swiftmailer.class%'
        public: false
        arguments:
            - '@acme.failover_transport'