A server physically located in a collocation facility in New York shared by left leaning organizations Rise Up Networks&May First/People Link was seized two days ago, 18th April, by the FBI turning up with a search warrant. The server belonged to the “European Counter Network“, an Italian group defining itself as “antifascist“, it provided email accounts, mailing lists, website hosting for activists and remailing to the public. It appears that an anonymous person sent more than 100 bomb threats over a period of months through the mixmaster remailer network to the University of Pittsburgh leading to numerous building evacuations while the police cleared all false alarms. No arrests have been made so far but the investigation remains open.
Riseup press release calls the server seizure an attack on free speech that has left artists, historians, gay rights groups, feminists and others without mailing lists and email accounts, various websites have also been taken offline as a consequence of the seizure. Riseup claims that while sympathizing with the University of Pittsburgh community they do not understand why the FBI has taken the server when “authorities knew that the server contained no useful information that would help in their investigation“.
Mixmaster remailers resemble the tor proxy network in that they do not log anything and work in chain mode, normally three servers in different jurisdictions are involved routing an email before being finally delivered to an inbox, however more servers could be involved if the sender specifies it in the settings. Mail servers running open source Mixmaster software remove header information to make it impossible finding out the sender, messages are deliberately held for some time to avoid time based attacks and it can take days or hours before an anonymous email is finally delivered.
A Mixmaster remailing server has been designed to make it impossible to trace emails back to the original source for the system to fail it would be necessary to seize all of the servers involved sending a message and recovering erased logs, assuming they ever existed. A new protocol called Mixminion is in development and intended to replace Mixmaster in the future.
More information: EFF article about remailer seizure