For what it's worth, I just ran into this bug today; and got here by googling "/var/lib/amavis/virusmail filled disk".
I was quite surprised and dismayed when I looked at df and found 0 bytes free on / and postfix (correctly) refusing mail with "uhh, sorry buddy, I'm full up! Come back later!". Also rather annoying because I discovered the default partition layout puts / before swap, so swap has to be destroyed before / can be extended... but that's another problem. I'll keep my eye on README.Debian and the NEWS file... -- Graham Cantin | (408) 890-7463
