[Evolution] how to flush outbox

[Cooper Yuan <cooperyuan gmail com>]

Hi everyone,

Recently I use evolution to connect with exchange server, perfect!!,
then I create a filter to auto forward the emails to my gmail box,
(exchange server has disabled auto-forwarding).
Evolution does process the incoming message, but it only put it into
outbox, that means, gmail box doesn't receive the forwarding mail,
until I flush the outbox manualy.

so, anybody could tell me how to flush the outbox automatically, I
can't create any new rule to achieve this. appreciate your help.

Cooper