Many of you who have been around the Outlook world undoubtedly know about .PST files. These are the main email database files that the Outlook client stores on local computer hard drives.

It turns out that Exchange (which is what you're talking to when talking to Office 365) can be configured to create a similar data file, the .OST file, which also stores a local copy of your email data.

So that brings us to four specific steps: Make sure you configure Outlook to download your entire email database; locate your .OST file; back up the .OST file; and know how to get information out of it if you need to.

Let's look at each of these steps in turn. I'm using Outlook 2013 in my examples, but Outlook 2010 and even Outlook 2007 work similarly.

Click the File tab in Outlook on the upper-left side of the Outlook window.

This brings you to the Account Information tab.

There are actually two ways you can do backups of your Outlook data: export and continuous copy. I tend to prefer keeping a continuously up-to-date copy, but I'll show you both. 

One-time export

One of our commenters colorfully pointed out there's an Export option on the File tab that will export an entire PST file. There is also a corresponding input option. Click Export and then Export to file. This process will often take a few hours to complete and while it will result in a .PST file (more usable to non-Exchange Outlook clients), you have to re-run the process every time you want a local copy.

Another reader mentioned, and this is worth pointing out, that PST files have size limits. Older Outlooks were limited to 2GB, while newer ones hover around 18GB.

On-going local copy

The approach I tend to use is creating on-going local copies on my various client machines. That's what I'll be describing through the rest of this guide.

If you don't see Account Information, click the Info menu item on the side of the page.

Next, click the Account Settings drop-down button and click the Account Settings menu item. This will display the email accounts dialog.

Note that you can also get to this via the Mail control panel, but since finding that is different on Windows 7, Windows 8, and Windows 8.1, I took the more consistent path for this guide.

Make sure the E-mail tab is selected, then click the little Change button. You'll see the Change Account dialog. This is where the magic takes place.

You need to make two changes (or at least make sure your settings match). First, be sure the Used Cached Exchange Mode is checked. Checking this means Outlook stores a copy of your Exchange data in a local .OST file. Next, be sure the slider is all the way to the right, so "Mail to keep offline" is set to All.

Click Next and Finish to close out of the dialog. Now, this is important: Don't quit Outlook, and keep it open and running overnight to download all of your data. I usually leave my Outlook open all the time, so I just let it run for a few days and then I'm completely sure that my data is copied to my local machine.

I also run Outlook on a few different machines, so I have multiple .OST file backups.

Now that you've set up your Outlook to download all of your data and you've given it a good, long time to cook, let's locate that file.

Follow the same set of steps as described earlier to get to the Account Settings dialog, but this time, click the Data Files tab. There, you will see the location of your data file and a convenient Open File Location button.

If you click the Open File Location button, you'll see a file. As you can see in the screenshot, my file is almost 8GB in size.

Now here's an important tip: Before backing this up, make sure Outlook is closed. You might want to check Task Manager to make doubly sure there's no Outlook process running.

Whatever you do, don't kill that process. Either reboot to make it shut down or be patient. I'd recommend rebooting if you want a clean, pristine backup copy.

Way back in 2006, I wrote an article about how to copy content from one .PST file to another. Although that article was probably about using Outlook 2003, the process is actually still the same. So click on over to OutlookPower to read the step-by-step guide.

Disclosure: OutlookPower.com is one of my personal website projects.

Outlook 2011 on the Mac doesn't work like any other Outlook I've ever seen. Outlook 2011 on the Mac stores a bajillion individual messages. I'm guessing this was to allow Spotlight to properly search the messages, but it's still a bit odd for an old-school Outlook user.

In any case, some of your data is there, in the form of individual messages. You don't appear to be able to tell Outlook 2011 to download all of your messages. I set up the Mac in the image above in early December, and Outlook 2011 seems to have only downloaded messages since that time.

While this isn't the best solution, you can actually open those .message files inside of TextEdit.

There is a better way, in the form of the Export item from the File menu. You can choose to export an Outlook for Mac Data File (.olm) and dump Mail, Contacts, Calendar, Tasks, and Notes. Unfortunately, the Windows and Mac export formats are not compatible. If you want to move from Mac to Windows or Windows to Mac with Outlook, your best path is using an Exchange server (or Outlook 365) in the middle.

My advice to you, though, is if you're an Office 365 user, you can get up to five free installs of Office, plus you can get "Office Anywhere". I'd recommend installing Outlook on a PC you can control for a few days, download the .OST file as described earlier, and back it up. It will save you a lot of hassle versus dealing with thousands of individual .message files.