Microsoft outlook 2010 stores all email address into a file and so the next time you type that name or email, it will autocomplete it for you. But some it become very frustrating, when you are typing an address in the ‘To’ field, and outlook doesn’t offer some of the addresses of your contacts. But sometime problem may be differ than that, when you didn’t know how to delete an Autocomplete Outlook address.
So, In this Microsoft outlook help blog, I’ll go through a few methods to fix the autocomplete problem in Outlook 2010. Find easy steps :
Step 1: First go to ‘Options > Mail > Send Messages > Use Auto-Complete list‘ and check that auto complete feature is enable or not, then run ‘scanpst.exe‘ to fix error and disable all outlook add-ons.
Step 2: Delete autostream.dat file and let Outlook remake it.
Step 3: Now go to ‘c:\users\name\appdata\local\microsoft\outlook‘ and rename the ‘RoamCache’ folder and let Outlook remake it.
After all, empty the autocomplete list, disabled autocomplete, closed outlook and re-enabled autocomplete feature. Now check that it is working or not. If it doesn’t work, follow these steps below to fix this Microsoft outlook problem .
First open a new message and go to ‘TO…‘ button and select the contact list in the right top corner from which you would like to add the contacts from. (Exp: Contacts folder, Global Address List, or a contacts folder in a Public folder.)
Now select the entire names or email accounts from top to bottom with hold the shift key and click on ‘OK’ close the dialogue and to return to the newly created message. Now close this message and click on ‘No’ when prompted to save changes. Also find here how to import .nk2 files into Outlook 2010.
I hope it will be work now.
Additional Tips:
If you are facing unknown email problem form unwanted or unknown sender, you can easily block the sender for sending email again. Find here easy methods to block a sender in Microsoft Outlook 2010.
Vtechsquad Inc. 
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Slickit.ca actually has a very good list of solutions for this problem and has since last year. Thanks Panth for posting a link to your blog on mine.
In older versions of outlook you could copy and rename nk2 file but no such thing in Outlook 2010 so how do you import cached contacts form old 2010 profile to new profile in outlook 2010 after you have changed from a pop account to a hosted exchange account which required you to recreate a new email profile?
View outlook recover. The tool is able to use modern methods of working with big outlook files. Utility can recover outlook data after different issues like as viruses, accidentally deleting, malware, hard disk failures.