The top of the SpamSieve Support page always has the latest information about compatibility and public beta versions.
It is important (see below) to update SpamSieve before updating macOS. To prevent macOS from updating itself without asking you:
Compatibility With macOS Updates
SpamSieve is developed using best practices, so it ordinarily continues to run normally when macOS is updated. That said, newer versions of SpamSieve are better at recognizing the latest types of spam—and contain many other improvements—so it’s best to run the latest version of SpamSieve that your Mac supports. You can easily update to the latest version by choosing Software Update… from the SpamSieve menu.
Compatibility With Apple Mail Updates
If you are using Apple Mail, you may have to update SpamSieve each time you update macOS. The reason for this is that SpamSieve uses a plug-in to integrate with Apple Mail. Each time Apple makes a change to Mail—which it does in most OS updates—there is a chance that the plug-in will stop working properly. Depending on the nature of the changes, an old plug-in running in a newer version of Mail could prevent Mail itself from working properly. (This has never actually happened with the SpamSieve plug-in, but we can’t rule out the possibility.)
Apple Mail Compatibility UUIDs
There are many Mail plug-ins, and Apple understandably cannot test all of them, let alone their interactions with each other. Thus, starting around macOS 10.6, Apple adopted a policy where each plug-in declares (using UUIDs) which versions of Mail it has been tested with. If a plug-in has not been tested with the version of Mail that you’re using, Mail will not load it.
The advantage to this system is that it makes Mail more reliable; it will not load any untested code. The disadvantage is that plug-ins like SpamSieve’s, which make only a few hooks into very stable areas of Mail, usually do not require any changes to work with the new version of Mail. When Apple updates Mail, we have to ship a new version of the plug-in, with the new UUID to prove that it was tested with that version; customers have to update SpamSieve and its plug-in; but the new plug-in has the exact same code as the old one (aside from improvements unrelated to OS compatibility). Nevertheless, the UUID system does improve Mail’s reliability and does not cause problems so long as customers are aware that they need to update SpamSieve (and any other plug-ins).
Timing of SpamSieve Update Availability
Typically we ship an update to SpamSieve the same day that Apple updates macOS. Some customers have asked why SpamSieve updates do not become available until after Apple has shipped a macOS update.
From macOS 10.1 through 10.8, we would test SpamSieve with pre-release version of the OS. If a SpamSieve update was needed for compatibility with the new OS, we would try to ship the update before that version of the OS was released to the public. This was possible because there was generally only one set of UUIDs for each public release of macOS.
With later releases of macOS 10.8 through macOS 10.12 (but not 10.10.3 through 10.10.5), there are new UUIDs for most pre-release versions of macOS. With this new policy, we still test with pre-release versions of the OS, so we can have a SpamSieve update ready to go, but we cannot actually ship it until after the OS update ships. The reason for this is that the compatibility UUID that we need to declare is not known until we have the final shipping version of the OS update. This is the system working as designed: the UUID certifies that we have tested SpamSieve on the exact version of the OS that is running on your Mac.
Update SpamSieve Before Updating macOS
The smoothest way to update macOS and SpamSieve is to update SpamSieve before updating macOS.
The reason for this is that if you update macOS first, and then open Mail, Mail will not load the old version of the SpamSieve plug-in. Without the plug-in loaded, you will not see the SpamSieve commands in Mail’s Message menu, and Mail will move good messages to the spam mailbox without consulting SpamSieve. If you get into this situation, quit Mail and then update SpamSieve. The next time you launch SpamSieve, it should detect that the plug-in was disabled by an OS update and reinstall the plug-in. If this does not happen automatically, you can choose Install Apple Mail Plug-In from the SpamSieve menu.
On the other hand, if you update SpamSieve before updating macOS, you will have the latest version of the plug-in when you launch the new version of Mail, so Mail will not disable it.
Using Pre-Release Versions of macOS
If you are using a pre-release version of macOS, it will likely have a different compatibility UUID than the shipping version of SpamSieve. There are several ways to deal with this: