MAJOR problem with address auto-expanding on 10.7.2's Mail

On my Mac mini running 10.7.2, I have Address Book configured to use a CardDAV server, with two separate accounts, one for personal and one for business contacts. Mail.app is doing a REALLY WEIRD thing right now, which is also extremely damaging, in that if I have a message addressed to or from the account name on the CardDAV account, Mail.app is treating it as an address group and expands it automatically. It's doing this not only on the To: line, but the From: line as well.

So, let's say my email addresses are a@example.com and b@example.com. If I send an email from a@example.com, the From line is replaced with the entirety of a@example.com's address book (which both appears as spam AND provides my ENTIRE ADDRESS BOOK to all the recipients). If I send an email TO a@example.com, then the recipient list is replaced with the entirety of a@example.com's address book as well. Sending a test email from a@example.com to b@example.com ends up sending a@example.com's entire address book to everyone in b@example.com's address book!

This is absolutely terrible and it means I can't use Mail.app until this issue is fixed. I have no idea how many mails I've sent with it like this, either, because it's not showing up like this in my sent messages - just in the occasional bounces I've received from remote spam filters.