Use System Preferences to add your Google account information to OS X.
To add your Google Account here, click the plus button in the lower left, then click Gmail in the list of new account options on the right. Whether you're using a regular @gmail account or Google Apps to run your Gmail through your domain (so your address looks like [you]@davidchartier.com), add your name, email address, and password on the sheet that appears, and click Set Up.
OS X will check with Google's servers and then display a sheet offering all the apps you can use with your Google account. As of this writing, those include email, calendar, Messages (via the Google Talk services), and Notes. All will be checked by default, so feel free to disable any you don't want.
If you do want to use some of your other Google Account services in OS X, though, this is a great way to make the magic happen.
When you add your Google account to this System Preferences pane, you can access all your other Google services, too.
Step 4: Set up Mail
Apple has made some strides when it comes to making Mail behave better with Gmail, but this working relationship still has a few catches.
Deal with archiving issues: One of Gmail's perks is a ridiculous amount of storage space, so Google has set it up to highly encourage archiving your email instead of having to make the decision to delete just some of it. After all, you never know if that rainy day will come next month or four years from now, and there's no harm in keeping tiny emails around in this age of ever-expanding storage space.
As of OS X 10.7 and 10.8, you might notice a new Message > Archive option in Mail's menu. You can also use View > Customize Toolbar to add an Archive button to the toolbars of Mail's main window and individual message viewer windows. Unfortunately, after two years and a number of bug reports filed with Apple, that button still doesn't work properly with Gmail.
Press the button, and it will create a new top-level folder called Archive where you can now store messages that you don't need in your Inbox, but that you want to be able to search later. This does not follow Gmail's behavior on the Web of moving messages to All Mail, which means that messages archived through Mail won't be stored in the place Gmail understands.
If you rarely sign into Gmail on the Web, you don't need to worry about this. But if you do--say, while on vacation away from your devices or using a different work computer--some of Gmail's built-in archive and search features won't know to look in the Archive folder that Mail created.
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