Comment 3 for bug 428814

The cron job should not be removed on package uninstallation, only on package purge as outlined in the debian policy - section 9.5:

    All files installed in any of these directories must be scripts (e.g., shell scripts or Perl scripts) so that they can easily be modified by the local system administrator. In addition, they must be treated as configuration files.

    [...]

    The scripts or crontab entries in these directories should check if all necessary programs are installed before they try to execute them. Otherwise, problems will arise when a package was removed but not purged since configuration files are kept on the system in this situation.

amavisd-new cron job should make sure that all its relevant binaries are available before running.