SpamSieve is supposed to launch when there are new messages to filter, i.e. when Mail downloads messages from the server and applies the SpamSieve rule to them. So if there are no new messages, it is normal for it not to launch when Mail launches.
If there are new messages and it’s not auto-launching, please check that:
- The latest version of SpamSieve is stored directly in the Applications folder, as shown here.
- You do not have any other versions of SpamSieve on your Mac. In particular, having an old version in the trash can prevent the new version from launching.
Ordinarily, if one of the above is a problem, the SpamSieve plug-in will tell you that it is not able to find and/or launch SpamSieve. If you don’t see such an error message, then the problem is likely that the SpamSieve rule is not being applied to any messages, so the plug-in doesn’t see any need to launch SpamSieve.