Three days ago, Microsoft randomly started rejecting mail from this list, apparently as well as several others, as some sort of anti-spam measure. This caused a lot of bounces and messages about members getting unsubscribed.
It affected hotmail.com, outlook.com, live.com and perhaps a few other recipient domains.
I've proactively resubscribed everyone that seems to have been affected.
If you find list mail being routed to your junk folder, please follow the link to choose "Trust this sender" which may help this kind of thing from happening again. I have no clue what caused it, and apparently neither does Microsoft.