Bill,

> Partially correct.  SMTP allows for allw recipients at a particular mail
> exchanger to be sent together.  The conversation would go something like
> this
   [details snipped]

Thanks for clarifying the sequence.

> 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.

What I experienced is, IMO, a broken mailer. No spam filtering was involved,
and there was only one address (probably deleted because the employee had
left the company) which was unknown. The mailer proceeded to return the same
error report (listing the actual failing address) for every subscriber in
that domain, although I believe all those with valid addresses got their
copies.

Alec's Red Cross case seems to differ only in that his list server and
target addresses all seem to be within the domain with the problem. That
might make the failing software easier to identify; all I could do was send
notes to the company postmaster (and some participants who I suspected might
be influential).

There does seem to be mail software out there that hasn't got a very good
sense of proportion when it comes to error handling.

Hal Keen