Microsoft Outlook's AutoComplete feature saves time by suggesting previously used recipient names and email addresses while you type in the To, Cc or Bcc fields.

For example, after you have previously emailed:

accounts@example.com

typing only:

acc

may cause Outlook to suggest the complete email address.

Many users assume these suggestions are stored in Outlook Contacts. That is not necessarily true. Outlook maintains a separate AutoComplete or nickname cache containing addresses learned from previous email activity.

This becomes particularly important when:

• replacing an old computer,
• reinstalling Windows,
• reinstalling Microsoft Office,
• creating a new Outlook profile,
• migrating Outlook to another computer,
• recovering data from an old hard disk,
• upgrading from Outlook 2007 to a newer Outlook version,
• restoring a user's previously used email addresses.

Older Outlook versions stored this information in a file with the extension:

.NK2

However, Microsoft changed the AutoComplete architecture beginning with Outlook 2010.

Understanding this difference is essential before attempting an Outlook AutoComplete migration.

1. What Is an Outlook .NK2 File?

An .NK2 file is an Outlook nickname cache file.

It contains information about recipients that Outlook previously used for AutoComplete suggestions.

An NK2 file can contain information such as:

• recipient display name,
• email address,
• SMTP address,
• Exchange addressing information,
• AutoComplete display information,
• previously resolved recipients.

The file is automatically updated as the user sends email.

Example

Suppose you previously sent emails to:

• accounts@company.com
• billing@vendor.com
• support@example.com
• john@example.com

Outlook may remember these addresses even when they were never manually added to Contacts.

This is why users sometimes migrate their PST file or Contacts and then discover that their familiar AutoComplete addresses are missing.

2. AutoComplete Is Not the Same as Outlook Contacts

This distinction is extremely important.

Outlook Contacts

Contacts are intentionally created and stored as contact records.

They may contain:

• name,
• email address,
• telephone number,
• company,
• designation,
• postal address,
• notes.

AutoComplete Cache

AutoComplete is primarily a history of recipients Outlook has previously resolved or used.

An email address can therefore appear in AutoComplete even though it does not exist in Contacts.

Consequently:

Backing up Contacts does not necessarily back up the AutoComplete list.

Similarly, importing Contacts does not always reproduce the exact AutoComplete experience that existed on the old computer.

3. Which Outlook Versions Use .NK2 Files?

The storage mechanism changed significantly after Outlook 2007.

Outlook Version	Normal AutoComplete Storage
Outlook 2003	.NK2 file
Outlook 2007	.NK2 file
Outlook 2010	New AutoComplete architecture; can import old NK2
Outlook 2013	New AutoComplete architecture
Outlook 2016	New AutoComplete architecture
Outlook 2019	New AutoComplete architecture
Outlook 2021/LTSC	New AutoComplete architecture
Outlook 2024/LTSC	New AutoComplete architecture
Microsoft 365 Classic Outlook	Modern AutoComplete architecture

Important: Outlook 2010 and later do not normally continue using the imported .NK2 file as their active AutoComplete database.

The .NK2 file is primarily relevant when transferring AutoComplete information from Outlook 2007 or earlier into a newer Outlook installation.

4. Where Is the NK2 File Located?

For Outlook versions that use NK2 files, a common location is:

%APPDATA%\Microsoft\Outlook

You can open this location easily.

Procedure

1. Press Windows + R.
2. Enter:

%APPDATA%\Microsoft\Outlook

3. Press Enter.

Look for a file ending in:

.nk2

Example:

Outlook.nk2

or:

Sales.nk2

The filename normally corresponds to the Outlook profile name.

5. Make Windows Show File Extensions

Windows may hide known file extensions.

This can cause confusion because:

Outlook.nk2

may appear simply as:

Outlook

In Windows 10 or Windows 11:

1. Open File Explorer.
2. Select View.
3. Enable File name extensions.

You can then confirm that the file actually ends in .nk2.

6. Back Up the NK2 File Before Doing Anything

Before importing, renaming or editing the NK2 file, create a backup.

For example:

Outlook.nk2

Copy it to another folder and name the backup something such as:

Outlook-NK2-Backup.nk2

Keep another copy on external storage if the AutoComplete list is important.

Do not work on the only available copy of an old NK2 file.

7. Find the Correct Outlook Profile Name

The NK2 filename used during migration should match the Outlook profile name.

To determine the profile:

1. Close Outlook.
2. Open Control Panel.
3. Search for Mail.
4. Open Mail (Microsoft Outlook).
5. Select Show Profiles.

You may see a profile such as:

Outlook

or:

Office

or another custom profile name.

If the profile is:

Outlook

the NK2 file should normally be named:

Outlook.nk2

before performing the legacy NK2 import.

8. Recommended Method: Import an Old NK2 File into Outlook 2010 or Later

This is the most important migration procedure.

Suppose an old computer running Outlook 2007 contains:

Outlook.nk2

and you want to restore those addresses into a newer version of Classic Outlook.

Step 1 – Close Outlook

Make sure Outlook is completely closed.

Check Task Manager if necessary and confirm that OUTLOOK.EXE is no longer running.

Step 2 – Determine the New Outlook Profile Name

Open:

Control Panel → Mail → Show Profiles

Make a note of the profile name.

Example:

Outlook

Step 3 – Rename the NK2 File

Rename the old NK2 file so that its filename matches the new Outlook profile.

Example:

Outlook.nk2

Step 4 – Copy the File

Press:

Windows + R

Enter:

%APPDATA%\Microsoft\Outlook

Press Enter.

Copy the .NK2 file into this folder.

Step 5 – Run the Import Command

Press:

Windows + R

Enter:

outlook.exe /importnk2

and press Enter.

Outlook should start and import the old nickname cache.

Step 6 – Test AutoComplete

Create a new email.

Start typing the first few characters of an email address that you know existed in the old AutoComplete list.

If the migration succeeded, Outlook should suggest the address.

9. Important Correction About /importnk2

A common mistake is using a command such as:

outlook.exe /importnk2 "C:\Backup\Outlook.nk2"

The standard Microsoft migration procedure is different.

The recommended approach is:

1. Copy the NK2 file into:

%APPDATA%\Microsoft\Outlook

2. Rename it to match the Outlook profile.
3. Run:

outlook.exe /importnk2

The command is therefore normally a one-time import operation rather than a command where the NK2 filename must be supplied as an argument.

10. Outlook Says It Cannot Find outlook.exe

Normally:

outlook.exe /importnk2

should work through Windows Run.

If Windows cannot locate Outlook, you may need to run Outlook using its actual installation path.

The location varies according to:

• Microsoft 365 installation,
• Office version,
• 32-bit vs 64-bit Office,
• Click-to-Run installation,
• MSI installation.

Rather than assuming a fixed Office path, locate OUTLOOK.EXE on the computer and run the switch against that executable.

Example structure:

"C:\...\OUTLOOK.EXE" /importnk2

11. What Happens to the NK2 File After Import?

Outlook 2010 and later use a different AutoComplete storage architecture from Outlook 2007.

Therefore, importing an NK2 file should be considered a migration process.

The newer Outlook version imports the information into its newer AutoComplete system rather than permanently operating from the legacy NK2 file.

This is why repeatedly copying an NK2 file into a modern Outlook installation is not the correct long-term management method.

12. What Is Stream_Autocomplete*.dat?

Administrators working with newer Classic Outlook installations may encounter files similar to:

Stream_Autocomplete_0_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.dat

in:

%LOCALAPPDATA%\Microsoft\Outlook\RoamCache

These files are associated with Outlook's newer AutoComplete architecture.

However, they should not simply be treated as equivalent to the old Outlook 2007 NK2 file.

Depending on the Outlook version and account architecture, AutoComplete information may also be maintained in the Outlook message store as hidden AutoComplete data.

Therefore, manually replacing Stream_Autocomplete*.dat files should be considered an advanced troubleshooting/migration technique rather than the first recommended method.

13. Where Outlook 2010+ Stores AutoComplete Information

Beginning with Outlook 2010, Microsoft changed the way AutoComplete information is stored.

Instead of relying on a traditional NK2 file, AutoComplete information can be maintained in an Outlook message-store structure associated with:

IPM.Configuration.Autocomplete

Administrators may also see local cache information in the Outlook RoamCache folder.

This architectural change explains why simply searching for:

Outlook.nk2

on a modern computer may return nothing even though AutoComplete is working normally.

14. Outlook AutoComplete Settings

Before assuming that AutoComplete data has been lost, verify that the feature is enabled.

In Classic Outlook:

1. Open Outlook.
2. Select File.
3. Select Options.
4. Select Mail.
5. Locate Send messages.
6. Make sure the following option is enabled:

Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines

If this setting is disabled, Outlook will not display the normal AutoComplete list.

15. How to Remove One Incorrect AutoComplete Address

Sometimes the problem is not a missing AutoComplete list but an obsolete or incorrect email address.

To remove one entry:

1. Create a new email.
2. Start typing the recipient's name.
3. Wait for the incorrect suggestion to appear.
4. Highlight the suggestion.
5. Click the X beside it, where available.

You can also use the appropriate keyboard deletion option in supported Classic Outlook versions.

Removing an AutoComplete entry does not necessarily delete an Outlook Contact.

16. How to Clear the Entire AutoComplete List

If the AutoComplete cache is badly corrupted, Outlook provides a method to clear it.

Go to:

File → Options → Mail → Send messages

Select:

Empty Auto-Complete List

Confirm the operation.

Warning

This removes the cached AutoComplete suggestions.

Do not perform this operation simply as an experiment on a user's computer if the existing AutoComplete list is valuable.

Back up or document important information first.

17. Command to Clear the AutoComplete Cache

Classic Outlook also supports the command:

Outlook.exe /CleanAutoCompleteCache

This can be useful when troubleshooting a corrupted AutoComplete cache.

Warning

This command is intended to clear the AutoComplete cache.

It should not be confused with:

Outlook.exe /importnk2

The commands perform completely different operations.

Command	Purpose
Outlook.exe /importnk2	Imports legacy NK2 AutoComplete data
Outlook.exe /CleanAutoCompleteCache	Clears the AutoComplete cache

Always verify that you are using the correct command.

18. ImportNK2 Does Not Work – Troubleshooting Checklist

If:

outlook.exe /importnk2

does not restore the addresses, check the following.

1. Outlook Was Running During the Procedure

Close Outlook completely before copying or renaming files.

2. Incorrect Profile Name

Check:

Control Panel → Mail → Show Profiles

The NK2 filename should correspond to the profile used for the import.

3. Incorrect Folder

The legacy NK2 file should be placed in:

%APPDATA%\Microsoft\Outlook

for the Microsoft-documented import procedure.

4. Wrong File Extension

Make sure the file is actually:

Outlook.nk2

and not accidentally:

Outlook.nk2.nk2

or:

Outlook.nk2.txt

Enable file extensions in File Explorer to verify.

5. Wrong Outlook Profile Opens

If multiple Outlook profiles exist, Outlook may start using another profile.

Check the default profile in:

Control Panel → Mail → Show Profiles

6. NK2 File Is Corrupted

An old NK2 file can become corrupted.

If the file cannot be imported normally, a specialist NK2 editor may help inspect or recover individual entries.

7. Import Already Occurred

The /importnk2 switch is intended for migration of legacy nickname-cache information.

After successful migration, manage AutoComplete through the newer Outlook storage system.

19. Using NK2Edit for Advanced Recovery

A well-known third-party utility called NK2Edit from NirSoft can inspect and modify Outlook AutoComplete information.

It can be useful when:

• an NK2 file needs inspection,
• obsolete addresses need removal,
• display names need correction,
• addresses need exporting,
• an NK2 file needs troubleshooting,
• AutoComplete data needs advanced migration.

Possible operations include:

• viewing cached recipients,
• modifying entries,
• deleting unwanted entries,
• exporting entries,
• importing entries,
• examining legacy NK2 information.

Security Recommendation

Download utilities only from their legitimate publisher.

Before manipulating business-critical Outlook information:

1. close Outlook,
2. back up the original data,
3. keep an untouched copy,
4. work on a duplicate whenever possible.

20. Can NK2Edit Work with Newer Outlook Versions?

NK2Edit was originally designed around NK2 nickname-cache files but also includes functionality related to newer Outlook AutoComplete formats.

However, Outlook's modern AutoComplete architecture is more complex than simply maintaining one .NK2 file.

For business-critical migrations, Microsoft's supported migration procedure should be attempted first.

Use third-party tools primarily when:

• normal import fails,
• the old data requires inspection,
• individual addresses need recovery,
• advanced editing is required.

21. Outlook AutoComplete Maximum Entries

Modern Classic Outlook versions generally maintain a limited number of AutoComplete entries.

For current supported versions such as Outlook for Microsoft 365, Outlook 2019 and Outlook 2016, Microsoft documents a limit of approximately:

1,000 entries

Once the cache reaches its limit, Outlook can use internal usage-based logic to remove entries.

Therefore, AutoComplete should not be considered a permanent address-book or CRM database.

Important email addresses should be stored as proper Contacts or in an organizational directory.

22. AutoComplete Should Not Be Used as a Contact Backup

Many users depend heavily on AutoComplete and assume that every suggested email address is safely stored in Contacts.

This can lead to problems during computer replacement.

If an address is business-critical:

Save it as an Outlook Contact or maintain it in an appropriate central address book.

AutoComplete should be considered a productivity feature rather than a permanent contact-management system.

23. Recommended Outlook Migration Strategy

When moving Outlook from an old computer to a new computer, consider backing up more than just the PST file.

Depending on the environment, review:

• PST files,
• Outlook Contacts,
• signatures,
• rules,
• templates,
• AutoComplete information,
• account configuration,
• local archives,
• custom stationery,
• Quick Steps where applicable,
• locally stored certificates,
• email account credentials,
• Outlook profiles,
• add-in configuration.

The exact items depend on whether the mailbox uses:

• POP3,
• IMAP,
• Microsoft Exchange,
• Microsoft 365/Exchange Online,
• another hosted email platform.

24. POP3, IMAP and Exchange/Microsoft 365 Can Behave Differently

Do not assume every Outlook account stores or synchronizes AutoComplete information in exactly the same way.

The migration method can depend on:

• Outlook version,
• mailbox type,
• Outlook profile,
• Windows user profile,
• Microsoft 365/Exchange environment,
• local data-store configuration.

For this reason, identify both the Outlook version and account type before performing a business-critical AutoComplete migration.

25. Important Note About New Outlook for Windows

The procedures in this article are primarily intended for Classic Microsoft Outlook for Windows, particularly when dealing with legacy NK2 migration.

Microsoft's newer Outlook applications have different architectures and should not automatically be treated as equivalent to Classic Outlook.

If you cannot find:

• traditional Outlook profiles,
• NK2 functionality,
• familiar Outlook command-line switches,
• expected Classic Outlook folders,

first verify whether you are using Classic Outlook for Windows or the newer Outlook application.

26. Practical Migration Example

Scenario

Old computer:

• Windows 7
• Outlook 2007
• hundreds of AutoComplete addresses
• NK2 file available

New computer:

• Windows 11
• Classic Outlook
• Microsoft 365 Apps

Recommended Process

On the old computer, obtain:

%APPDATA%\Microsoft\Outlook\Outlook.nk2

Copy the file safely.

On the new computer:

1. Configure Classic Outlook.
2. Start Outlook once if necessary.
3. Close Outlook.
4. Check the Outlook profile name using Control Panel → Mail.
5. Rename the NK2 file to match that profile.
6. Copy it into:

%APPDATA%\Microsoft\Outlook

7. Run:

outlook.exe /importnk2

8. Open a new email.
9. Type several known recipient names.
10. Verify that the old suggestions appear.

Do not delete the original NK2 backup immediately. Keep it until the migration has been thoroughly verified.

27. Important Backup Recommendation for IT Administrators

Before replacing an employee's computer, do not rely solely on:

"All mail is in Outlook, so everything will come automatically."

AutoComplete information can be especially important for employees who communicate with hundreds of clients, vendors or customers but have never saved those addresses as Contacts.

Before decommissioning an old PC:

• confirm the Outlook version,
• identify the mailbox type,
• check the Outlook profile,
• preserve legacy NK2 files when applicable,
• preserve relevant Outlook data,
• verify Contacts,
• test AutoComplete on the replacement PC,
• retain the old backup until the user confirms successful migration.

Frequently Asked Questions (FAQ)

Q1. What is an NK2 file in Outlook?

An NK2 file is a legacy Outlook nickname-cache file used to store recipient information for AutoComplete suggestions.

Q2. Which Outlook versions use NK2 files?

Outlook 2003 and Outlook 2007 are the main versions associated with direct NK2 storage. Outlook 2010 and later changed the AutoComplete storage architecture but can import older NK2 data.

Q3. Where is my Outlook NK2 file?

For legacy Outlook installations, check:

%APPDATA%\Microsoft\Outlook

Q4. Why can't I find an NK2 file in Outlook 2016, 2019, 2021, 2024 or Microsoft 365?

Because modern Outlook versions no longer normally use an NK2 file as their active AutoComplete storage mechanism.

Q5. What command imports an old NK2 file?

Use:

outlook.exe /importnk2

after preparing the NK2 file according to the Outlook profile and placing it in the appropriate Outlook folder.

Q6. Should I specify the NK2 filename after /importnk2?

Microsoft's documented migration procedure normally uses:

outlook /importnk2

after the correctly named NK2 file has been placed in the Outlook configuration folder.

Q7. Does importing a PST restore AutoComplete?

Not necessarily. AutoComplete and Outlook Contacts/PST contents should not be assumed to be identical.

Q8. Are AutoComplete addresses the same as Contacts?

No. An email address can exist in AutoComplete without being saved as a Contact.

Q9. Can I transfer Outlook AutoComplete to another computer?

Yes, but the correct method depends on the Outlook version and account type.

Q10. What is Stream_Autocomplete*.dat?

It is associated with the newer Outlook AutoComplete cache architecture and can commonly be encountered in Outlook's RoamCache area.

Q11. Where is the Outlook RoamCache folder?

A commonly encountered location is:

%LOCALAPPDATA%\Microsoft\Outlook\RoamCache

Q12. Can I simply copy Stream_Autocomplete.dat to another computer?

It may work in some specific migration scenarios, but it should not be treated as a universal or preferred migration method because newer Outlook AutoComplete storage can involve the Outlook message store as well as local caching.

Q13. What is IPM.Configuration.Autocomplete?

It is associated with the hidden Outlook message-store mechanism used for AutoComplete information in newer Outlook architectures.

Q14. How do I enable AutoComplete?

In Classic Outlook go to:

File → Options → Mail → Send messages

and enable:

Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines.

Q15. How do I delete one incorrect email suggestion?

Start typing the recipient and remove the unwanted suggestion using the X/delete mechanism provided by Classic Outlook.

Q16. How do I clear the entire Outlook AutoComplete list?

Go to:

File → Options → Mail → Empty Auto-Complete List

Q17. Is there a command to reset AutoComplete?

Classic Outlook supports:

Outlook.exe /CleanAutoCompleteCache

Use it carefully because it clears AutoComplete information.

Q18. Can a corrupted NK2 file be repaired?

Sometimes. Third-party utilities such as NK2Edit can inspect and modify NK2 data. Always retain an untouched backup first.

Q19. What is NK2Edit?

NK2Edit is a utility developed by NirSoft for viewing, editing, exporting and managing Outlook AutoComplete/NK2 information.

Q20. Is AutoComplete a reliable backup of email addresses?

No. Important addresses should be stored in Contacts, a directory, CRM or another managed contact system.

Q21. Why did some old AutoComplete addresses disappear?

Possible causes include:

• cache limits,
• Outlook profile replacement,
• Windows reinstallation,
• corrupted AutoComplete data,
• migration to another computer,
• cache reset,
• account/profile changes.

Q22. How many AutoComplete entries can Outlook hold?

Microsoft documents approximately 1,000 entries for several modern Classic Outlook versions.

Q23. Does AutoComplete synchronize with Outlook on the web?

Do not assume that Classic Outlook and Outlook on the web use exactly the same AutoComplete list. Their suggestion systems can differ.

Q24. Should I delete my old NK2 file after importing it?

Keep a backup until you have confirmed that all required AutoComplete addresses migrated successfully.

Q25. Can I import an Outlook 2007 NK2 file into a newer Outlook installation?

Yes. Legacy NK2 import is one of the purposes of the /importnk2 switch in Classic Outlook.

Best Practices Summary

For a safe Outlook AutoComplete migration:

1. Identify the old and new Outlook versions.
2. Determine the email account type.
3. Back up the original NK2 or AutoComplete information.
4. Never modify the only available copy.
5. Confirm the Outlook profile name.
6. Use Microsoft's /importnk2 method for legacy NK2 migration.
7. Verify AutoComplete after migration.
8. Keep important email addresses as proper Contacts.
9. Use third-party tools only when necessary.
10. Never clear the AutoComplete cache before determining whether valuable recipient information needs to be preserved.

Conclusion

The Outlook .NK2 file is an important legacy component of Microsoft Outlook's AutoComplete system. It was primarily used by Outlook 2003 and Outlook 2007 to remember previously used recipient names and email addresses.

Beginning with Outlook 2010, Microsoft changed the AutoComplete architecture. Newer Classic Outlook versions can still import legacy NK2 information, but they do not normally continue operating from the NK2 file.

For an old-to-new PC migration, the safest approach is therefore to identify the Outlook versions and profile first, preserve the original data, and use:

outlook.exe /importnk2

when migrating a legacy NK2 file into a compatible newer Classic Outlook installation.

Most importantly, AutoComplete should never be treated as a substitute for a proper Contacts database. Business-critical email addresses should be stored as Contacts or in an appropriate centralized directory so that a damaged Windows profile, Outlook profile or AutoComplete cache does not result in the loss of valuable recipient information.

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