Sorry, Mail. My fledging relationship with you was just killed thanks to OS X Mavericks.
TidBITS confirmed what I suspected the first time I launched Apple Mail after upgrading to Mavericks on Tuesday -- Gmail and Apple Mail are no longer friends. It was always a tenuous relationship to begin with. As writer Joe Kissell pointed out, you had to do a bit of a runaround to bring Gmail onto your Mac without hogging your entire hard drive. But now Apple Mail is trying to act like Gmail, and in the process is running into numerous syncing issues, ballooning the size of your mailbox and forcing you to activate Gmail's "All Mail" on Apple Mail, lest you want to see your messages constantly reappear in your inbox.
While I managed to escape Kissell's issue of having all three of my Gmail accounts download everything again, my smart mailboxes proceeded to break. It kept telling me I had numerous unread messages, but nothing was showing in the mailbox. In the screencap above, you can see the smart mailbox selected but nothing in the mailbox itself. When I checked the rules for that smart mailbox, it had a lot of blank rules that weren't there before. I deleted them, but the smart mailbox still didn't work.
Your best solution for now? If you're a huge Gmail user on the Mac, it's worth checking into a third-party client, such as Sparrow, Postbox or Mailplane. I've gone back to the standard web client for Gmail for now, until I can try the beta of Mail Pilot. Sorry, Mail, our brief fling is over.
Are you guys seeing issues with Apple Mail in Mavericks? Let us know in the comments.