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.