PureMessage 5.5.9 introduced a version of the anti-virus engine (4.47) that is not backwards compatible with any version of PureMessage prior to 5.5.9. For this reason, Sophos no longer supports tarball upgrades from pre-5.5.9 versions to any version except 5.5.9. From 5.5.9, you can update to a more recent 5.x version.
As of the 5.5.9 release, versions prior to 5.5.9 are not available for download from the Sophos tarball archive (http://pmx.sophos.com/tarball/). If you attempt to install or upgrade from a previously saved, pre-5.5.9 tarball, content extraction and matching will not work as intended. If you are using an unsupported version of PureMessage, contact Sophos Technical Support to discuss upgrade options.
For currently supported versions, see the PureMessage Versions and Platforms Support Policy. For instructions on upgrading from a tarball, see Upgrading PureMessage from a Tarball Distribution.