The Install Plug-In button in Settings ‣ Apple Mail ‣ Setup installs (or updates) the plug-in that SpamSieve uses to integrate with Apple Mail. If the plug-in is successfully installed, you will see the SpamSieve commands in Mail’s Message menu. For more information, see the Setting Up Apple Mail and Why don’t I see the SpamSieve commands in Apple Mail’s Message menu? sections of the manual.
Installing the plug-in also causes SpamSieve to install a launch agent so that the SpamSieve application will launch and be available for filtering when Apple Mail launches. On macOS 13, this will cause macOS to show a notification saying Background Items Added. This is normal. SpamSieve will show up under System Settings ‣ General ‣ Login Items ‣ Allow in the Background, and you should not uncheck it there, as that would prevent it from working with Apple Mail. The login item will automatically be removed if you select Disable spam filtering in Apple Mail.
A macOS 13 bug can cause the system to incorrectly and/or repeatedly report Background Items Added when SpamSieve didn’t change anything. This may stop after a safe boot. If it doesn’t, please click here to enable some additional debug logging and send in a diagnostic report after the next time the problem occurs.