I had a user in Salt Lake Perl Mongers who seemed unable to get messages to go through. He checked his spam filters, I checked all of the administrative configuration I could find... nothing seemed unique to his situation aside from the situation itself.

Finally he unsubscribed and re-subscribed. If I recall, that seemed to fix things.

However, you may check the administrative settings for the problematic user's email address and compare them to others who are successful. It may be that you turn up something just by looking at the various check-boxes. Another thing to consider is, do you have the "not metoo" box checked? If so, you won't see your own posts, but others will. And finally, check things like his "nomail" setting (which may get triggered if you have a few bounces).

Also, it is possible to configure a PM mailing list with things like content filtering, so you may want to look at those settings too.

I wish I could be more specific, but there are lots of configuration options under the "Administration" pages, and some of them are not real well documented. But as a first step, do drop the user from the list and ask him to re-subscribe, or to switch to using some other email account.