> Anyone has an idea how to achieve this situation? :
> mailbox_size_limit = 0
> message_size_limit = 0
> and mail gets delivered normally?
> I don't think this is wise, but it's a customer request and the customer is
> a very stubborn one.
Before you troubleshoot any further, are you delivering to virtual mailboxes?
In any case, increase verbosity (or debug level) and check the logs for clues.