[FC5] "Sender address rejected" for just about all email now

["Stuart Murray-Smith" <eight32@xxxxxxxxx>]

Hi list

Package mix:
Postfix-2.2.2-2
Cyrus-imapd-2.2.12-6
Amavisd-new-2.4.2-2

I'm getting:

Sender address rejected: Domain not found

for just about all inbound email.

Time to switch off spam filters and reassess filtering policy? Or has
one of the blacklists that I'm not aware of unlearned spam patterns?

Please advise :-)

Smee