my view on this: you have a message on the queue, which is "hard to
deliver" (say the other party has network problems or misconfigured
network, you can't connect to it, but you don't get notified about it
either, so you gotta wait timeouts).
if you have q_r_i_o set, then this message will sooner or later get to
the top of the queue, then immediately stops other messages from being
delivered. ok, holds them up...
then you have the next message to the same dest, the next, and yet
another. suddenly your other mails just can't get out, because these
hold the queue run up, badly.
if, however, q_r_i_o is not set, then these messages will be hit, of
course, by the queue runner, but what's the chance you *always* get
these as first ones ?
i look at this option pretty much the same way as retry rules. not
really a good, but you get the point...
