blackholes.uceb.org was an antispam RBL that shut down in 2008, but as with all RBLs, they tend to find their way into mail server configs and then ossify there.
It looks like whoever ran uceb.org decided that two years was enough and to let the domain lapse. Yesterday, the domain’s registrar put the domain in a “pending delete or resale” status:
Created On:21-Mar-2002 11:13:47 UTC
Last Updated On:27-Mar-2010 08:19:20 UTC
Expiration Date:21-Mar-2011 11:13:47 UTC
Sponsoring Registrar:Network Solutions LLC (R63-LROR)
Status:CLIENT TRANSFER PROHIBITED
Registrant Name:Pending Renewal or Deletion
and then, as is pretty standard operating procedure in cases like this: they wildcarded the domain’s DNS.
That means for anbody who was still referencing blackholes.uceb.org in their mailer config, it wasn’t doing much damage (or good) since July, 2008…until yesterday. Then they probably started rejecting ALL email because all addresses within *.blackholes.uceb.org now return true….