"There was some hard system failure, and system hardware and operating system keepers needed a few hours to get that level sorted out. Then we have slowly enabled email subsystems one at the time to verify correct operation. Back to enjoying the flood :-)"
More information
http://lwn.net/Articles/274079/
Kernel filesystem failure instead?
Jeremy,
mgalgoci says that it was a filesystem failure after a kernel oops (http://lwn.net/Articles/274409/) -- do you have any more information on that?