Right, thank you so much. I thought there was something like delay_warning_time that could make c) a workable choice. In fact, now I remember setting that up on my production server (when I had one a year ago) so senders would get notified their mail was still queued:
But then this creates an additional bounce message and Charles makes a valid point:
Again... the only unit responsible for sending actual bounce messages is the SENDERS MTA. Your (receiving) MTA should only either ACCEPT (if so, NEVER generate a 'bounce' later), DEFER or REJECT.
However, I remember setting delay_warning_time for the benefit of local domain senders, so they would get notification of delayed delivery. If this is added, bounces to non-local domain sender would be a side effect of deferring over-quota messages.
At any rate, thanks. I now have a good picture of what all the options are.
-- Gary V