[Mailman-Users] Emails Not Being Delivered to All on List

[Bruce E. Breeding]

Symptom:  some list recipients are not receiving their emails from the list [Four resulting questions below]

In attempting to determine WHY the above symptom is occurring, I contacted my web service provider.  We're getting to know each other well due to frequency of communication.  ;)
He seems to be trying hard to help me out.  
Here is his latest helpful hint:

**** Web service provider's suggestions start here ...
"I happened to catch some of your mailing lists trying to deliver a little while ago and caught an example of why you sometimes have problems. This is a good example of a list doing a partial delivery because it stalls at a bad address which causes delays etc for other addresses that are on the same list and this particular address you have on for lists currently unable to finish delivery.......................

Getting MX record for <domainname>.org (from local DNS server, may be cached)...   Received a SERVER FAILURE response.

This should be treated as an ERROR (per RFC974), and the E-mail delivery should PROBABLY be retried later.

Here is what has been delivered so far, anything with a "Y" has been delivered, anything without is stalled because of the bad email address which doesn't allow the mailserver to continue.

<and here, he inserted a list of email addresses from the list, each prefaced with either
YY, YN, NY, or NN>

<I presume he's getting this from (hopefully appropriate) log.>

<He even suggested I check email addresses at http://www.dnsreport.com (which I have, although I had already checked one by one to be sure the addresses were correctly entered.  I have most of them in a contact list and so there was little doubt in my mind of their accuracy.)>
**** Web service provider's suggestions end here ...

So, my Question(1):  Is it normal to expect a substantial amount of non-delivery to valid email addresses?  I would presume, "No."  But, in case I'm really missing the obvious, here, please enlighten me.

Question (2):  Is it normal for Mailman to "stall", or "stop" processing the rest of a list because of one bad email?  (I'm asking that somewhat rhetorically, expecting an emphatic "NO" answer.)

Question (3):  Please explain the YY, YN, NY, and NN indicators [preceding the email addresses] from the above referenced log.  I understand that YY means the email was delivered to the recipient, and NN means the email was NOT delivered to the recipient.  But, YN and NY?  maybe yes, maybe no?

Question (4):  Do you have further suggestions as to troubleshooting the reason that some of these addresses did not receive the email?  Are there specific places (other logs in mailman) I should ask him to look that could give further information?

Thank you for you help.
Bruce