In rare cases, two bugs in macOS can cause the training commands to fail. The SpamSieve - Train as Spam command doesn’t move the message to the Spam mailbox, and an error such as:
SpamSieve Apple Mail Plug-In [Train as Spam] Error Training: Mail got an error: Can’t get mailbox "INBOX" of account "<account>".
is reported in Console. The SpamSieve - Train as Good command doesn’t move the message to the inbox, and an error such as:
SpamSieve Apple Mail Plug-In [Train as Good] Error Training: Mail got an error: An error of type 8 has occurred.
is reported in Console.
To make the training commands work and prevent these errors:
Choose Settings from the Mail menu.
Click on the Accounts tab.
Make sure that each account has a unique name. You can edit the Description field for an account to change its name so that it is different from the other accounts’ names.
For example, you may have both an active POP account and an inactive IMAP account for the same username and server. These might both be called “Account A”. In this case, you could rename the inactive one to “Account A Inactive” so that the names are different.
If you rename an inactive account, Mail will ask if you want to enable the account. Click Leave Disabled.
If renaming the accounts doesn’t help, please follow the instructions in the Sending in an Error Report section.