correction (Re: Transport involvement in Delivery Acknowledgements)

In a very slightly earlier message, I wrote this:

> Yes.  I think there is general agreement, even on this list, that support
> for delivery acks in receiving UAs/delivery agents must be optional.
By "this list" I meant the ietf-ack list, not the ietf-822 list.
(I didn't notice that the message was addressed to ietf-822 instead of
ietf-ack.)

Keith