warning about using swiftmail with file spool and lazy loading

email/spool.rst

@@ -141,3 +141,21 @@ You can also set the time limit in seconds:
 Of course you will not want to run this manually in reality. Instead, the
 console command should be triggered by a cron job or scheduled task and run
 at a regular interval.
+
+.. caution::
+
+    When you create a message with SwiftMailer, it generates a ``Swift_Message``
+    class. If the ``swiftmailer`` service is lazy loaded, it generates instead a
+    proxy class named ``Swift_Message_<someRandomCharacters>``.
+
+    If you use the memory spool, this change is transparent and has no impact.
+    But when using the filesystem spool, the message class is serialized in
+    a file with the randomized class name. The problem is that this random 
+    class name changes on every cache clear. So if you send a mail and then you
+    clear the cache, the message will not be unserializable.
+
+    On the next execution of ``swiftmailer:spool:send`` an error will raise because
+    the class ``Swift_Message_<someRandomCharacters>`` doesn't exist (anymore).
+
+    The solutions are either to use the memory spool or to load the 
+    ``swiftmailer`` service without ``lazy`` option (see :doc:`/service_container/lazy_services`).