[exim] route_list, randomize and failover

Hi all,

I have four server and balance between them with the randomize option.
Now I also want some kind of failover which I try to realize with the
following syntax:

route_list = *
mx1:mx2:mx3:mx4:+:mx2:mx3:mx4:+:mx1:mx3:mx4:+:mx1:mx3:mx4:+:mx1:mx2:mx4
randomize byname

(all in one line)

Why doesn't this work or how can I accomplish this? I want to cover all
server outages and still have the randomizing.

thx in advance,

    Markus Meyer