The service Riseup.net used by activists is currently having disk problems which prevents email being sent.

"The current status is this: all users can read old email, but half of the users are unable to receive new email. To check if you are currently unable to receive new mail you can login at user.riseup.net and click on "Email". If your "storage host" is set to cormorant.riseup.net, then all your incoming mail is getting pooled up waiting to be delivered. It will get delivered as soon as we can bring a replacement server online, hopefully within the next few hours."

That was posted late on 26th and things are still not back to normal. Many lists will be affected, such as the Critical Mass  cm-london@lists.riseup.net.