I was using SpamSieve just fine for many months on Catalina, then today I rebooted after an OS update & SpamSieve had been disabled by the OS. I tried re-installing from the DMG of SpamSieve-2.9.39 from the website, but when I drag the application into the Applications folder, I get the following:
“SpamSieve” cannot be opened because the developer cannot be verified.
The first few times I tried this, it complained that the application is “damaged” so I re-downloaded it a couple of times.
I fixed this by going into the System Preferences > Security & Privacy > General tab & clicking on “Open Anyway” next to the message about Spam Sieve not being from an identified developer.
I’m posting this, just in case others run into the same thing (or if for some reason the app really should be signed & I have a bad copy).