To install amavisd-new just follow these instructions.
/debian/stretch/main///install/index.html Interface between MTA and virus scanner/content filters Debian 9.0 mail 1:2.10.1-4
sudo apt-get install amavisd-new
Which will install amavisd-new and any other packages on which it depends.
|Description||Interface between MTA and virus scanner/content filters|