If you poke around, I'm sure you can figure out how to disable amavisd-new using tmpfs. (tmpfs was removed in 4.0.3 or 4.0.4 release because of this bug)
Just another word on reasons to upgrade (besides the obvious issue with security updates, stability, etc)... as with any open-source software, the ability to get support from the community on something less than the current version is very limited and dependent on the generosity of the community. Good support is less and less likely the further away you get from the release date of the version you are running when it isn't current.
If you were paying for the network edition, then of course that is a different situation