> I think (and here the L-Soft mavens may correct me) that when there are 
a
> group of subscribers on the same list, some of the processing for 
delivery
> of their individual copies may be combined in such a way that the whole 
set
> of recipients is known to the destination email system, in connection of 
its
> handling of any given copy. The problem arises when that system reports 
a
> delivery failure for one of the addresses, but sends back that report in
> connection with all of them.

Partially correct.  SMTP allows for allw recipients at a particular mail 
exchanger to be sent together.  The conversation would go something like 
this (from your mail relay's perspective):

-> HELO mail.domain.com
<- 250 Pleased to meet you
-> MAIL FROM: <[log in to unmask]>
<- 250 Sender OK
-> RCPT TO: <[log in to unmask]>
<- 250 Recipient OK
-> RCPT TO: <[log in to unmask]>
<- 550 Unknown user
-> RCPT TO: <[log in to unmask]>
<- 250 Recipient OK
-> DATA
<_ 354 Send body, end with CRLF.CRLF
-> (Send messageending with CRLF.CRLF)
<- 250 message accecpted for delivery

The second, unknown user should not affect delivery to any other 
recipients, unless one side or the other is using a broken mailer or has 
misconfigured one that normally behaves.  In this day of nearly ubiquitous 
spam filters, it's not hard to find ones that don't behave well.  They 
could even return a failure after the DATA phase if there are too many 
unknown/rejected users, or for almost any other reason.