Responding to Mark Sapiro:

If the sendmail log says the post is sent to the user in question, Mailman delivered it to sendmail for the user, and it's not Mailman's problem.

I agree, it's not clear whether this is a sendmail or a Mailman
problem. However, I have never, AFAIK, failed to receive an email
from my provider's domain, unless that email originated from this
specific Mailman list. That seems suspicious, doesn't it? Of course,
I may be missing other emails that I am not aware of. Not getting
something you did not know to expect is not an easy problem to trap.

Since I now know that I am not receiving all emails from this list,
possibly the problem is common but not often detected because the
severity varies, list by list.

Mailman's 'smtp' log. This will tell you how many recipients the post was sent to.

My provider has checked Mailman's smtp log. It shows emails were sent
to all subscribers.

Also check if the list is personalized. You may not even have personalization options on your Non-digest options page which means that OWNERS_CAN_ENABLE_PERSONALIZATION is not set to Yes in mm_cfg.py and delivery is almost certainly not personailzed.

I'm not certain, but I think this answers your question: the list
does have digest options available to the users.

Check the settings of VERP_DELIVERY_INTERVAL and SMTP_MAX_RCPTS if any in mm_cfg.py

I looked, there are no settings for these in Mailman/mm_cfg.py. There
don't appear to be any site-specific settings here.

You should also see this in the sendmail log, i.e. that the mail was delivered to sendmail in one transaction.

The provider has given me sendmail logs that I have analyzed. I was
able to identify entries in the sendmail log that correspond with
mailing list transmissions that were not received by the subscriber
(me).

Sendmail claims it sent the mail to the users who didn't get it. I recognize the symptoms say that Mailman is somehow involved, but I don't see how.

I recognize the difficulty of this particular problem.

Thanks, Alan