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?