It sounds strange, but Mail doesn't always work on the Mac. Yesterday at 3:30PM I watched the notification center tell me about the 20 messages just coming in (at once?) that I didn't see at the top of my mail list when I went to look. That's because they were supposed to be there between 8:30AM and 10:00AM that morning, which is where they were.
At a one of our TR Care™ customer's business today, similar problems. A executive assistant was perplexed why a message she read at 7:20 that morning wasn't in her inbox on there Mac's Mail app, but was in her inbox on her iPhone. And on her Mac, it was in "All Mail" (they use Google Apps for Business, just like I do). Quitting and restarting Mail put the message back where it belongs.
But you shouldn't have to do that. Mail is just supposed to be. And I shouldn't get a spinning pinwheel on a top-of-the-line 1.7GHz MacBook Air with 8GB of RAM and 512GB of the fastest flash available.
I've been dealing with bugginess like this for a long time, probably since 10.6, and through the subsequent kitties: Snow Leopard, Lion, Mountain Lion, and even development software I'm not supposed to talk about. Yesterday was the final straw. I missed a critical, critical email from a customer that I was waiting for.
I did something pretty extreme: Backed up (three ways), then formatted and restored, to a clean Mac OS X 10.8.5. Updated. Launched a new, empty Mail, configured it with my account settings...... Same problem. This was not good. And I'm not going to go after Google on this one. My email works great in the web interface, which I don't like to use.
So this morning I do something I have been avoiding totally and completely: I found a new Mail application: Postbox. This app rocks. It adds a lot of features Outlook users miss, like "send later". But what matters is that it just works. And it's lighting fast. I've been using it all day, and I had probably about a 5 minute learning curve, which is good because I've been able to get back to work. My email is flowing smoothly between all my devices, which is even better.
I don't think the problem is with Mail per se. iCloud email seems to run well enough in Apple's Mail app. I think it's just that thinking about, investigating and building in support for other email servers with unique and powerful services like Google Apps' Gmail and even Exchange just really isn't there yet. I hope they fix this in the next year or two, but for now I'm ready for Postbox on my Mavericks. The $10 that Postbox cost is the best money I've invested in software since Omnifocus.