Previous recipients list stepped on by SpamSieve 2.9.21 under OS X 10.10.5?

I upgraded to 2.9.21 today, checked Mail -> Message to make certain the SpamSieve options were still in the menu to confirm that the plug-in had been installed, and then tried to compose a mail message. To my surprise, the addressee did not autocomplete. I tried opening the Mail -> Window -> Previous recipients window, but nothing happened. Even after sending some messages, the window will not open.

I have three active mail accounts on this client: a Exchange IMAP account, an OS X POP account, and an POP account with my ISP.

Is anyone else seeing this odd behavior?

I’ve not heard of that behavior before. SpamSieve does not do anything with Mail’s Previous Recipients list. Perhaps the database is damaged. There may be errors mentioning it in the Console.