The following was posted on the C-Command blog:
DropDMG 3.0.7, EagleFiler 1.5.4, and SpamSieve 2.8.6 have been tested with Mac OS X 10.7. If you haven’t already updated to the latest versions of these applications, now is the time to do so. There is currently one known issue to be aware of:
SpamSieve and Lion’s Apple Mail
*Summary: After updating to Lion, Apple Mail will not automatically launch SpamSieve. After launching Apple Mail for the first time, you must manually launch SpamSieve once (e.g. by clicking its icon in the Dock). Thereafter, Apple Mail will automatically launch SpamSieve, and everything will behave normally.
If you’re updating from Snow Leopard, the first time you launch Apple Mail under Lion it may disable SpamSieve’s plug-in. While the plug-in is disabled, Mail will not auto-launch SpamSieve, you will not see the SpamSieve commands in Mail’s Message menu, and Mail will move all new messages to the Spam mailbox without consulting SpamSieve.
For most users this will not be a problem, as a few new messages are easily dragged back to the inbox. (There’s no need to train them as good, since SpamSieve didn’t think they were spam.) However, if you’d like to prevent any messages from being misfiled, you can, before updating to Lion, open Mail’s Preferences window, go to the Rules tab, and uncheck the SpamSieve rule. This will temporarily disable it.
To get SpamSieve and Mail working again, just launch the SpamSieve application (e.g. by clicking on its icon in the Dock). It will detect that the plug-in has been disabled and automatically re-install it. (If, for some reason, this does not happen automatically, you can choose “Install Apple Mail Plug-In” from the SpamSieve menu.) If you had unchecked the SpamSieve rule, go back to Mail’s preferences and check it to re-enable SpamSieve.