Postbox ostensibly does not support a public chat forum or group. Their given reason, (which I personally disagree with) is that advice from non-experts might harm the Postbox user experience and so they don't support such a thing.
So this forum is not from Postbox-inc, it's from another interested party.
While it may be that the Postbox product is moribund, the maintenance of this discussion group is a separate issue.
I'm hoping both the group maintainer and the Postbox company both wake up a bit. I'd hate to have to change email clients again.