It's a simple two part edit to the amavisd-new script. I have this running on my kolab-16 installation (Ubuntu 18.04 LTS) and it solves several problems caused by amavis.

The attached zip contains that revised file. Please feel free to diiff it and include as you see fit.

The main issue it fixes is that Spamassassin can be configured to use the RelayCountries plugin, but then amavis deletes the headers. This fix adds those headers to the reserved list, and then an additional fix corrects the library additions needed to run that plugin.

There are complaints all across the interwebs about this issue. After seeing there was no fix, I dug in and fixed it myself. Amavis has had no active work since 2004.