We have taken the tough decision to remove the Subscribe by Email plugin by WPMU Dev from our servers.
This plugin allowed visitors to enter an email address and receive updates from class blogs whenever they were updated. In recent months our server monitoring shows that this plugin has been increasingly exploited to enable spammers to access our servers. The plugin itself does not support Captcha (the system that prevents Bots) from accessing sites that they should be excluded from. We were able to implement a solution on our servers that prevented bot access through the use of a "Captcha" plugin on our servers that blocked spam behind the scenes. However, this plugin was recently sold on to a developer with intentions that appear less than ethical (see this Wordfence blogpost). The plugin itself has now been removed from the Wordpress plugin directory.
This leaves us no option but to remove the plugin from all our sites and this has now been done.
We will look for a suitable and secure alternative.