Sometimes, after you try to enable the SpamSieve plug-in, Mail will report “Incompatible Plug-ins Disabled” (even though the version that you installed is compatible) or the installed plug-in will not appear or not stay checked in the Manage Plug-ins… sheet. Such problems generally mean that something is wrong with Mail’s private plug-ins folder, the place where it stores an extra copy of plug-ins that you have installed and enabled in Manage Plug-ins…. For example, this folder often gets messed up when migrating to a new Mac or restoring from backup.
Since the private plug-ins folder only stores duplicate copies of plug-ins that are also in the public folder (/Users/<username>/Library/Mail/Bundles/), you can safely delete the private folder without losing any data, and Mail will then let you enable the plug-in. Please follow the instructions below for the version of macOS that you’re using:
macOS 11 and Later
To delete Mail’s private plug-ins folder:
/Users/<username>/Library/Application Support/Mail/Plug-ins/
Quit Mail.
Open your user Library folder as described in How can I open the Library folder?.
Open the Application Support folder, then the Mail folder.
Drag the Plug-ins folder to the trash. If macOS won’t let you delete the folder, e.g. because it says you don’t have permission, try renaming it to Plug-ins Disabled instead.
If you were able to delete or rename the folder, launch Mail and click on Manage Plug-ins… in Settings ‣ General to enable the SpamSieve plug-in. You can skip the rest of the steps below.
Otherwise, you will need to temporarily turn off System Integrity Protection (as instructed below) so that Mail will let you delete the folder. This may also be necessary before you can empty the trash.
You may want to print these instructions, write them down, or open them on another computer or device so that you have access to them while rebooting your Mac.
Restart your Mac in Recovery Mode. The steps for doing this depend on which type of processor your Mac has:
From the Utilities menu at the top of the screen, choose Terminal.
Type csrutil disable and press Return. (This is what turns off SIP.) If it asks Allow booting unsigned operating systems and any kernel extensions for OS, enter n.
Click on the Apple menu and choose Restart.
You can now repeat Steps 2–4 to delete the Plug-ins folder.
Launch Mail and click on Manage Plug-ins… to enable the SpamSieve plug-in.
Repeat Steps 7–10, typing csrutil enable instead, in order to turn SIP back on.
macOS 10.14 and macOS 10.15
Ask Mail to Reset the DataVaults
In most cases, asking Mail to reset the DataVaults folder will work. To do this:
Quit Mail.
Open the Terminal app in the Utilities folder.
Enter this command:
defaults delete com.apple.mail EnableBundles
and press Return.
Launch Mail and then quit it.
Enter this command in Terminal:
defaults write com.apple.mail EnableBundles -bool YES
and press Return.
Delete the DataVaults Folder
If asking Mail to reset the folder doesn’t work, you can delete the DataVaults folder without losing any important data, and then Mail will recreate it when you try to enable the SpamSieve plug-in, hopefully fixing the problem:
You may want to print these instructions, write them down, or open them on another computer or device so that you have access to them while rebooting your Mac.
Click on the Apple menu and choose Restart…. Hold down the Command and R keys so that your Mac will reboot in Recovery Mode. This will enable you to temporarily turn off System Integrity Protection (as instructed below) so that you have access to delete the protected DataVaults folder.
From the Utilities menu at the top of the screen, choose Terminal.
Type csrutil disable and press Return. (This is what turns off SIP.) If it asks Allow booting unsigned operating systems and any kernel extensions for OS, enter n.
Click on the Apple menu and choose Restart.
The folder:
/Users/<username>/Library/Containers/com.apple.mail/Data/DataVaults/
needs to be dragged to the trash. (See How can I open the Library folder? if you have trouble finding it.)
Launch Mail and click on Manage Plug-ins… to enable the SpamSieve plug-in.
You can then repeat Steps 2–5, typing csrutil enable instead, in order to turn SIP back on.