It looks like the new version of Airmail (build 519) introduces a bug that prevents it from working with SpamSieve on macOS 10.14. Airmail is now built with the macOS 10.14 SDK, but it does not declare a NSAppleEventsUsageDescription in its Info.plist file. This means that the system does not prompt the user to let Airmail communicate with SpamSieve and there is no way to give it this permission in the Automation section of System Preferences.
I recommend that Airmail users not update to Airmail 3.6.3 until this is resolved. The previous version of Airmail should still work because macOS 10.14 does not enforce the NSAppleEventsUsageDescription requirement for apps built with the macOS 10.13 SDK.
If you’ve already updated to Airmail 3.6.3 or later, and don’t have backup of Airmail 3.6.1 that you can revert to, one option would be to run Apple Mail in the background and use it to filter your mail with SpamSieve until Airmail is working again.
It looks like Airmail 3.6.4 adds the NSAppleEventsUsageDescription. However, it is still not working with SpamSieve on my Mac. When I choose “SpamSieve - Train as Spam,” Airmail launches SpamSieve, but SpamSieve does not receive any communication from Airmail (nothing shows up in SpamSieve’s log).
I expected that I would get a prompt from the system asking if I wanted to allow Airmail to control SpamSieve, but there was no prompt, and Airmail does not appear in System Preferences ‣ Security & Privacy ‣ Privacy ‣ Automation. And the AppleScript that Airmail is trying to run is failing with errAEEventNotPermitted.
Please post here if you notice anything different. I have reported this to the Airmail developers.
At this point, my guess is that Airmail needs to declare the com.apple.security.automation.apple-events entitlement since it is now using the hardened runtime. Without that entitlement, the system won’t allow it to communicate with SpamSieve, or any of the other apps that it integrates with. I have reported this to the Airmail developers and hope that they will add it soon.