I've just fixed this problem by replacing amavis-postfix with a backport of amavisd-new from
http://www.backports.org/debian/
Incidentally, I had real trouble installing this, as it wouldn't install various bits of itsself and then complained about missing files during the configuration process.
I eventually found someone who had experienced the same problem and who told me to purge the package using dpkg (dpkg --purge amavisd-new)