they should still receive monthly notices because these are sent as Special Notices, but I have found several instances where these have been bounced for several months or even longer. However, there is no "Bouncing" Status badge against these members in the Members List.
The mechanism that sets the Bouncing status can have lots of leeway in it, depending on the response code from the receiving service and frequency of occurrence.
I have concluded - perhaps wrongly - that where a member has selected Special Notices Only a message bounce will not be automatically further investigated by system - generated Bounce Probes.
No bounce probes are sent until they've been changed to Bouncing status. Meanwhile message deliveries continue and are monitored, possibly leading to Bouncing status. I don't think the type of subscription figures into it other than changing the rate at which messages are sent to that member.
If my belief is correct then I think this configuration is wrong; any bounced message should be followed up by bounce probes and the member badged accordingly.
I'm not certain what the current status is for any given response code. It used to be that many codes would cause the mechanism to advance to Bouncing immediately. This has been backed off as too aggressive for some response codes, based on email service behavior.