for some users that are not paying for our services in time, we would
like to block their account from receiving new emails. To let them a
final chance to pay I would rather use :defer: than :fail: so other
servers will try to deliver the message again later.
But a very important part would be to customize the defer or fail
message at smtp-time, because otherwise the users as well as our
technical support wouldn't be able to distinguish for what reason a
message hasn't been accepted by our mailserver.
