Outlook 2010 Auto Complete Cache Corruption

A while back, I had written a fairly complete article on restoring the Outlook auto complete cache and working with .NK2 files.  Well, just when we thought that we could all return home in a victorious Napoleonic march, our enemy defeated, our pals down in Redmond reminded us all of Waterloo.

Outlook 2010, has completely done away with .NK2 files and has unfortunately replaced it with a system that seems far from bulletproof. My recent experiences, along with thousands of others on the net, suggests that the new system is rather prone to corruption and cannot be easily fixed.

But first, in a bid to dispel some of the misinformation out there, let me briefly go over how the new system works.

Users first using Outlook 2010, or syncing it to an active sync device are immediately going to notice a new set of contacts called suggested contacts that has been auto created upon opening a new mail account.  Inside, they will quickly notice that it is populated with email addresses of people whom they may not even remember having contact with. One soon realizes that this is a contact list that is being generated based on emails that have been sent out and incoming emails that do not have a proper contact in Outlook.  This idea, actually, seems to make a lot of sense as it is an easily backed up way of managing all of those contacts that were previously stored in the Auto complete cache’s .NK2 file.  It would be easy to come to the conclusion that this is the same thing as the .nk2 cache and this is what populates it. This, however, is wrong.  While it does store similar information, there is in fact a new .dat file that has taken the place of the venerable .NK2 file, but now has a GUID attached to it that is,ostensibly,l generated based on characteristics of the profile.  Apparently, this file can be transferred around just as was done with .NK2 files, but you would have to rename it with the exact same corresponding GUID for it to work. Definitely, not as slick as the simple renaming it to the name of the profile as in previous versions of Outlook. This won’t, however, be an issue if you have transitioned to Exchange 2010 as the file is actually stored on the Exchange server and synced back down to the client allowing for your auto complete cache to follow you as part of a new profile on a second computer.  Again, this is a great idea if it weren’t for the apparent instability of the file itself.

I have been able to find very little information on the causes of the issue, and nothing at all from Microsoft, but it appears that this file syncs itself back to the Exchange server upon closing Outlook. However, if for some reason, Outlook does not close cleanly, it appears to cut short the write of this data which subsequently corrupts the file. It is at this point extremely difficult to correct the situation. 

The file is located at %appdata%\Local\Microsoft\Outlook\RoamCache  and will be the file labeled Stream_Autocomplete_<GUID>.dat. As I stated, I have yet to come across anything that immediately fixes the issue in all cases, but here are a few things to try.

First, to verify that you are having the issue in question, you will a.) have an auto-complete cache that isn’t working and b.) a Stream_Autocomplete file that is 0 bytes in size and stays that way despite adding data to the autocomplete cache during a session. With this issue, your cache will work during the duration of Outlook being opened, but then when you close outlook and reopen it, all of the data is lost. 

Anyway, here are some methods that are known to work.

1. Delete the auto-cache data through the provided menu interface provided in Outlook.  To do this,

1. In Outlook 2010, Click the File menu and Select Options.


2. In the Outlook Options window Click the Mail tab.


3. Scroll down roughly halfway until you see Send messages. Uncheck the Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines box.


2. The second method is to manually delete the file mentioned above. Make sure and close outlook before you do this, as it will have a lock on the file and prevent you from doing so otherwise.  This method is really the same as what happens above, but some people have actually reported that the file did not delete as it was supposed to from the GUI.

3. Rename then whole Roam Cache folder and reopen outlook.

4. Disable cached exchange mode and try methods 1,2, or 3 above and repeat until the file starts to grow in size.

And please, if anyone else has any more information on the actual root cause of this, please comment below so we can all get through this.  There are many possible culprits, but I suspect that over time we will find that it is one particular add-on or program that is causing the vast majority of cases.



  1. Chokdii, none of your solutions worked in my case but I finally found a solution that does work. Close Oulook. Copy a working Stream_Autocomplete_.dat file from another computer, change the portion of the filename to that of the "broken" one. Open Outlook. Fixed!!

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  8. This comment has been removed by the author.

  9. you don't need to do too hard..just go ''c:\users\name\appdata\local\microsoft\outlook' and rename the 'RoamCache' folder and let Outlook remake it. and alternate method is ..Autocomplete Not Working In Outlook 2010? Now Fix It..

    1. Panth if you would have read the article you would have noticed that your advice is exactly as stated as one of the solutions.

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