I was incredibly excited about how easy this is. Microsoft Outlook stores most of its data in a rapidly ballooning file called a .pst. Your drop-down menu email addresses (or autocomplete email addresses) are backed up differently than most data in Microsoft Outlook. They’re stored in a .nk2 file.
To backup your .nk2 file and your dropdown email addresses, just navigate to it, make a copy and paste the copy in your preferred backup location. Your .nk2 file will be located here:
C:\Documents and Settings\*USERNAME*\Application Data\Microsoft\Outlook\Outlook.NK2
Of course, you’ll need to replace “*USERNAME*” with your actual username, and, in some instances, you may not be able to see your Application Data folder. If that’s the case, go to your username folder and click Tools → Folder Options → View → Select Show hidden files and folder → Apply → OK.
You should now be able to navigate to your .nk2 folder and back it up. If you want to see the text of that file so that you can copy down all your dropdown menu email addresses, you’ll need to run a special utility. I haven’t used the following program, but NK2View seems to fit the bill. Good luck!
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It seems like everything you want to do in Microsoft Outlook is harder than it should be, but if there’s a will, there’s generally a way to get it done. Here’s a how to guide for backing up your Microsoft Outlook folders and emails in a .pst file. You can scroll down if you’d like to see the how to guide for opening your already backed up .pst files in Microsoft Outlook. This guide applies to XP (and, I believe, Vista – although I haven’t confirmed that):
1. Go to File import and export:
2. Select export to a file:

3. Select “Personal Folder File”

4. Expand the (+) sign where it says “Inbox” and click on the folders you want to back up:
5. Click browse to select where you would like to save the .pst file. Hit “OK” when you’ve picked out the right location, then, click “Finish”

6. Your backup file is now saved! You can now go into your outlook and delete the folders from your inbox (since you have the files backed up).
If you ever need to reopen the files that you backed up, follow these instructions
How to open .pst files
1. Go to File import and export:
2. Select “Import from another program or file” and hit next.
3. Scroll down in the option list and select “Personal Folder File”, then click next

4. Locate the file by browsing to it, and click, “open”. You can leave the options unchanged (see below), and click “next”

5. Highlight the area where you would like the “folder” to appear in your inbox, then click finish.

6. Your folder will be restored, and you’ll now be able to access all those old emails.