It seamed ORDB (ORDB.org was shut down on December 18, 2006) changed since yesterday their behaviour, and instead of timing out, they are blocking all IP addresses (every e-mail server queried is being reported as an open relay). If your mail infrastructure uses ORDB, the sender will get a message like this one (this is an example blacklisting the Gmail servers): 

Technical details of permanent failure:
PERM_FAILURE: SMTP Error (state 13): 550-Message rejected because ag-out-0708.google.com [72.14.246.240]:20081 is
550-blacklisted at relays.ordb.org see ordb.org was shut down on December 18,
550 2006. Please remove from your mailserver.

The reason behind this active behaviour change is not clear yet, if we get more information we will update this entry here.