Here's a warning from Jim Hamm about Mavericks, "If you use a Gmail account for your emails, and use the Mac Mail app to download your emails, according to this article you may have a problem. Trying to read and understand the article almost gave me a headache, so I hope you do better.
"I use Gmail, but on the web server. I haven't downloaded emails into Mac Mail or any other program, such as Thunderbird, for years. I just prefer to use Gmail's web server program. It simplifies my email life.
Jim closes with, "If you've installed Mavericks, and download your email into Mac Mail, you may want to take a peak at this article. Good luck...(grin)"