when i try to install dbmail onto a debian sid system...

I added:-

deb http://debian.nfgd.net/debian/ sid main

to a file in /etc/apt/sources.list.d (called dbmail.list).

Apt complained that the repository was not secure so I added:-

[allow-insecure=true] 

after deb.

Then I did an apt update, and apt install dbmail, but I was then
told that libzdb9 was uninstallable.  

Where do I find libzdb9 - I could find libzdb7 but not 9 in the 
contrib section of the above repository?

In the main debian repositories there is only a version of libzdb9 
for arm64.

David