Read this post from Dr. Drang concerning bugginess of Mail in OS X Mavericks (and mentions in passing issues with iCal, Address Book, and Messages).
That, in fact, is the most annoying thing about this mess. Apple has broken the covenant. The deal was that I get a mail client that isn’t fancy but works, and in return I don’t complain about a lack of features I’ll never use. It’s a simple arrangement that’s worked for nine years, and now it’s all this.
It just works.