[Mailman-Users] What causes an "Uncaught bounce notification" ?

Hi;

I have a subscriber who successfully posts to my GNUMailman list ( 2.1.13 ).

However, every once in a while one of her emails gets bounced to me, the listowner, as an "Uncaught bounce notification".

How can I figure out what she is doing to cause this?

Thanks much in advance for any info

Steve