Same issue here
Hi,
I've exactly the same issue here on 7 machines - all installed from scratch:
Debian 4.0 Etch AMD64-Bit, DELL R200
Plesk 9.0.1 with Postfix, Dr. Web, DNSBL, SpamAssassin, DomainKeys and SPF
Disabled Dr. Web and restaring Postfix did not solve the problem.