>   * Auto-Delete= Yes,Full-Auto,Delay(18),Max(2),Probe(27)
>
> My understanding is that this should result in any bouncing
> address being cleared down within a month. However, those
> annoying forwarded bounces have been appearing for more than
> three months. Again, does this suggest that I've set some
> parameter wrongly?

You probably should reduce the value in Probe.  What it means is
that about 1/27 of the subscribers are probed each day that a
message is posted.  Since you post only once a week, it will
take 27 weeks, not days, to probe everyone.

The tradeoff is that more resources are required to send messages
used as probes, as a separate copy is required for each address
being probed.

> I also have
>
>   * Renewal= 3-monthly,Delay(7),Probe

I would eliminate this.  There's no reason to do both active
and passive probes.