stopped receiving incoming mail for valid mailboxes

[jeremy-fleming]

Release 6.0.9_GA_2686.F7_20101115224621 F7 FOSS edition

My server all of a sudden stopped handling incomming mail at 0418 saturday morning. There are no apperant errors in the logs. If an email is addressed to an invalid mailbox postfix returns the appropriate 550 error. otherwise valid mailboxes return
"dsn=4.0.0, status=deferred (host mail.domain[76.xxx.xxx.xxx] said: 451 Please try again later (in reply to end of DATA command))"

i dont see any exceptions in the logs and all services appear to be running. this is a production server and has been down for 70+ hours now

please help

Jeremy