The site, ORDB.ORG, which provides an automated way of blacklisting third-party email servers that spammers use to send mail, was shut down on December 31, 2006.
If you still configured the ORDB.ORG in the Exchange Realtime Block List (RBL) settings, some external domains may not send e-mails to your Exchange Server. When Exchange server receives inbound connections from these problem domains, upon the RCPT TO: command, Exchange would attempts to query ORDB server for verification of the sending domain. This leads to the ORDB server shutting down meaning that the Exchange server fails to respond to the "RCPT TO:" command in a timely manner. This results in SMTP conversations timing out.