NT Security is a wide open discussion about Windows security issues. This is the corollary to NT Bug Traq, run by the same folks. Lastname, Firstname in the body of the message. This list is designed to take notable events covered by the Security Focus web site on a weekly basis. Topics include BugTraq, jobs, incidents, pools, and events.
Risks is a digest that describes many of the technological risks that happen in today's environment.
They also have a news group and digests available. The list is moderated and digested. There are plenty of mailing lists out there that relate to the topics we are all interested in. Some have very high volume, and others don't. I suggest using a mail filter to seperate inbound mailing list traffic to specific folders so you don't get crushed by volume.