What Is an OLK File? One Item From an Outlook for Mac Mailbox, Saved on Its Own
An OLK file is a single message, contact or attachment that Outlook for Mac wrote as its own file. Jump to your question below, or read it through in order.
An OLK file is a single message, contact or attachment that Outlook for Mac wrote as its own file. Jump to your question below, or read it through in order.
An OLK file is one item out of an Outlook for Mac mailbox, saved as its own file. One message, one contact, one attachment. A busy mailbox becomes hundreds of thousands of them.
That is the surprise. Windows Outlook packs the mailbox into one PST file. Outlook for Mac scatters it, and every piece gets an OLK extension.
Outlook for Mac never adopted the single-file mailbox. It writes every item separately into a fixed folder tree, and the extension says which kind of item it is.
An email becomes an olk15Message. A contact becomes an olk15Contact. Send that email an attachment and the attachment gets an OLK file of its own.
So an OLK file is a fragment, not the mailbox. Finding one in a backup is like finding page 40 of some book: real, readable, mostly useless on its own.
.olk14Message, .olk15Message and about two dozen moreOutlook.sqlite, in the same profileAn OLK file is not an Outlook address book. Several large file directories say it is, and that claim has been copied across the web for years. This page carried it too, until today.
The address book is another format on another platform. We cover it at the OAB file format, and it has nothing in common with OLK files beyond four letters.
One message with two attachments already writes four files. Multiply that by a few years of mail and the arithmetic gets uncomfortable.
Outlook spreads attachments over 256 numbered folders for exactly this reason, because no folder wants a million files in it.
This is why the profile folder is enormous and why Spotlight struggles with it. A 30 GB Outlook for Mac profile is not one 30 GB file. It is hundreds of thousands of small ones, and copying it takes far longer than its size suggests.
Two unrelated things carry this name. On the Mac, OLK is a file extension. On Windows, Olk is a folder name. Nothing is shared between them except the letters.
Ask the web what OLK means and you get two answers mixed together, usually on the same page. They come from different operating systems and different decades.
The name ends in something like .olk15Message. It sits inside an Outlook profile folder. Sections 3 to 7 are about this.
You found folders called Olk or OLK1A2B while hunting for attachments. Section 8 is about that.
| OLK files, on a Mac | Olk folder, on Windows | |
|---|---|---|
| What it is | File extensions | A folder name |
| Written by | Outlook for Mac | Outlook for Windows |
| Holds | Mail, contacts, events, attachments | Temporary copies and cache |
| Safe to delete | No. That is your mailbox | Usually yes |
| Covered in | Sections 3 to 7 below | Just section 8 |
The rest of this page treats those Mac files as its subject, because they are the real format. Section 8 clears up the folder.
Two families. Outlook for Mac 2011 writes olk14 names, Outlook 2016 for Mac writes olk15. Within one family, the ending after the number tells you what that file holds.
The number is the product version, not a year. Outlook for Mac 2011 is internally version 14, so its files say olk14. Outlook 2016 for Mac is version 15.
Yes, that is off by one from the year on the box. Microsoft numbered the Mac releases to match the Windows ones, and nobody has renamed them since.
There is no olk13 or earlier. Outlook for Mac 2011 was the first Outlook on the Mac since 2001, and it replaced Entourage, which shipped with Office 2001, X, 2004 and 2008. An Entourage identity holds no OLK files at all.
| Outlook for Mac 2011 | Outlook 2016 for Mac | Holds |
|---|---|---|
.olk14Message | .olk15Message | Outlook's own record of one email |
.olk14MsgSource | .olk15MsgSource | The complete original message |
.olk14MsgAttach | .olk15MsgAttachment | One attachment |
.olk14Contact | .olk15Contact | One contact card |
.olk14Event | .olk15Event | One calendar appointment |
.olk14Task | .olk15Task | One task |
.olk14Note | .olk15Note | One note |
.olk14Folder | .olk15Folder | One folder in the folder list |
.olk14Category | .olk15Category | One color category |
.olk14Recent | .olk15RecentAddresses | Addresses you typed before |
.olk14Signature | .olk15Signature | One email signature |
Watch the attachment row. Outlook for Mac 2011 wrote .olk14MsgAttach and Outlook 2016 for Mac writes .olk15MsgAttachment. The word grew four letters between versions, and several directory sites still list the short spelling for both.
There are more. .olk14MailAccount, .olk14Pref and .olk14Group turn up in a 2011 identity. None of them matter unless you are rebuilding one.
The capital letters in the middle are cosmetic. macOS does not care whether you write .olk15message or .olk15Message, and directory sites shout them in capitals. The mixed-case spelling is what Outlook actually writes.
One email can land as three OLK files. The MsgSource file is the real message, raw and complete. The Message file is Outlook's own record of it, and every attachment gets a third file.
Outlook for Mac does not keep an email in one piece. It splits the parts so the message list can be drawn without reading every message in full.
So the practical rule is short. Want the mail back? Take the MsgSource files. A MsgSource file is real mail in standard format, and plenty of tools read it.
Big file directories say Message holds the header and MsgSource holds the body. An open-source tool that parses the bytes says the opposite: that Message file is a body blob carrying no headers at all.
“Complete native MIME with all headers, body, and structure intact.”
Both camps agree on the part that decides what you do next. MsgSource carries the complete message. Message does not.
Take the OLK file everybody agrees about and the argument stops mattering.
Two quick checks tell you which you are holding. A MsgSource file opened in any text editor starts with readable mail headers, lines like Received: and Subject:.
A Message file shows recognisable words scattered through binary noise. If you can read the headers, you have the useful one.
OLK filenames are random UUIDs, and the three files belonging to one email do not share one. The link lives in Outlook.sqlite. Without that database the OLK files are a heap, not a mailbox.
This is the part that catches people out, and it is the reason copying OLK files out of a backup rarely ends well.
Not a subject. Not a date. A UUID, thirty-two hexadecimal characters with dashes, generated when Outlook saved the item.
The 11 in that path is a folder number, nothing more. Attachments are spread across 256 folders numbered 0 to 255, which keeps any single folder from filling up.
Now look for the message that attachment belongs to. Its file carries another UUID, in another folder, with nothing in either name to connect the two.
“There is no direct way to figure out which message in the Messages folder is associated with according to the name & location of a given attachment file.”
The joins live in Outlook.sqlite, in the same profile folder. A table there records which message owns which source file and which attachments, all by internal identifiers.
Lose that database and the OLK files survive, but nothing knows what belongs to what. That is the difference between a mailbox and 200,000 numbered binaries.
So back up the profile folder, never a folder of OLK files. Copying Messages on its own gives you items with no index, no folder structure and no way to tell drafts from sent mail.
Time Machine handles this correctly because it takes the entire folder. Copying one subfolder by hand does not.
There is one honest exception, and section 4 already named it. A MsgSource file is one complete message in standard format, so it stands up on its own even with no name worth reading.
Outlook for Mac 2011 keeps them in an identity under Documents. Outlook 2016 for Mac and Microsoft 365 keep them in a profile under Library, inside a sandboxed group container.
The folder moved between versions, and it moved for a reason. Apple tightened what applications may write outside their own sandbox, so Outlook moved into a group container.
The 2011 wording is an identity. The 2016 wording is a profile. Same idea, different name. Outlook can hold more than one of either.
UBF8T346G9 is Microsoft's developer team identifier. It looks like a mistake and it is not.
| Subfolder | Holds |
|---|---|
| Messages | One Message file per email |
| Message Sources | The complete originals, spread over numbered folders |
| Message Attachments | Attachments, across 256 folders numbered 0 to 255 |
| Contacts | Contact cards |
| Events | Calendar appointments |
| Folders | The folder list itself |
| Recent Addresses | Addresses you typed rather than saved |
| Signatures | Email signatures |
~/Library is hidden in Finder. Hold Option while opening the Go menu and Library appears, or use Go to Folder and paste the path. The folder has been hidden since OS X Lion and nothing you do in Finder reveals it permanently.
Other macOS oddities land on our macOS error fixes page.
Only the Outlook for Mac that wrote it, and only with the OLK file left where it is. Double-clicking one on its own achieves nothing. A text editor is the honest fallback, and it works on exactly one of the three kinds.
Every file directory that answers this question says the same thing, and most of them are guessing. Here is what actually reads one.
macOS has no application registered for OLK extensions, so the file gets no icon and no default handler. Choosing Outlook manually in Open With does not help either.
Outlook has no command for loading one item from disk. It expects to find items through its own database, in its own profile. An OLK file dragged onto the icon is outside all of that.
The text-editor trick is worth thirty seconds. Open a suspect file in TextEdit. Readable headers mean a MsgSource file, and recoverable mail. Binary noise with the odd readable word means a Message file, and section 4 explains why chasing that one is wasted effort.
On Windows, Olk is a folder name rather than a file type. Old Outlook versions wrote opened attachments to a hidden OLK folder. The new Outlook keeps its entire profile in one called Olk.
If you arrived here after finding a folder rather than a file, this section is your answer and the rest of this page is not about you.
| Era | What was called OLK | Where |
|---|---|---|
| Outlook 2000 to 2003 | A hidden folder holding opened attachments | Under Temporary Internet Files, named OLK plus random characters |
| Outlook 2007 to 2021 | Nothing. The name was retired | The same folder became Content.Outlook |
| The new Outlook | The entire profile folder | %LOCALAPPDATA%\Microsoft\Olk |
So the name came back. Twenty years after Outlook 2003 dropped it, the rebuilt Outlook for Windows puts its cache, its settings and its offline mail under a folder called Olk.
Open an attachment without saving it first and Outlook writes a copy somewhere hidden, so the virus scanner can check it before anything else does.
That copy is supposed to disappear when you close the message. It often does not, which is why people go looking for the folder in the first place, usually after closing a document they had been editing for an hour.
Windows records that location in the registry, in a string value named OutlookSecureTempFolder under the Outlook Security key for your version.
The old page you may have landed on listed nine registry paths under the heading Location of OLK files. Those paths point at this Windows temp folder. They have nothing to do with the Mac files that are actually called OLK, and that mix-up is repeated across most of the results for this search.
Chasing an error in the rebuilt client instead? Our new Outlook error fixes cover the ones people meet most.
Move the profile folder, never a selection of OLK files. Outlook for Mac imports a 2011 identity only into an empty profile, and Windows Outlook cannot read either one.
Section 5 explained why a folder of OLK files travels badly. This is what travels instead.
| Route | Good for | Catch |
|---|---|---|
| Copy the entire profile folder | A new Mac, same Outlook version | Outlook must be closed. Half a copy is worse than none |
| Export to an OLM archive | Mac to Mac, version to version | Outlook for Mac reads OLM. Outlook for Windows does not |
| Re-add the account | Any Exchange, Microsoft 365 or IMAP mailbox | Only downloads what is still on the server |
| Drag messages to Finder | A handful of important emails | Writes EML files, which almost anything reads |
Server mail is not really trapped. If that account is Exchange, Microsoft 365 or IMAP, its OLK files are a local cache of something the server still holds. Adding the account on the new machine rebuilds them. Only On My Computer folders are local-only, and those are the ones to worry about.
Microsoft is strict about this one, and the rule surprises people who try it on a working machine.
An Outlook for Mac 2011 identity imports only into an empty Outlook profile. Microsoft states it plainly, and the import refuses if the target profile already has mail in it.
So the order matters. Create a fresh profile, import the old identity into that, then merge or move what you need.
There is no Microsoft tool that converts OLK files to PST. The supported routes all go through a mail account rather than through the OLK files.
Connect the same account in Outlook for Windows and mail arrives as an OST, the Windows cache of that server mailbox. Our guides to the PST file format and what an OST file is cover what you get at that end.
Export hitting a wall instead? Outlook has its own numbered export failures. Our fixes for coded Outlook PST failures cover them, including the export size limit that stops large mailboxes halfway.
Outlook for Mac almost never blames one OLK file. It says the database needs rebuilding, or shows empty folders. Any fix works on that profile rather than on files inside it.
The OLK files are usually fine. What breaks is the index that knows where they are, and Outlook reports that in language that points nowhere useful.
| What you see | What it usually means | Start with |
|---|---|---|
| Folders look empty, files are on disk | The database lost its index of them | Rebuild the profile |
| Search finds nothing | Spotlight never indexed the profile | Re-index that folder |
| Outlook asks to rebuild on every launch | The rebuild is failing partway | A fresh profile, then copy the data in |
| Attachments missing, messages fine | The attachment files moved or were pruned | The server copy, if the account is online |
| The profile folder is enormous | Deleted mail that never got compacted | Empty Deleted Items inside Outlook |
MsgSource files out of the copy you made, open them in a text editor and keep what matters. It is slow, and it is the last resort for a reason.Never delete individual OLK files to reclaim space. The database still lists them, and Outlook then reports items it cannot open. That turns one full disk into a broken mailbox.
Delete mail inside Outlook instead, empty Deleted Items and let the profile shrink on its own.
Binary, undocumented, one item per file, indexed by a SQLite database in the same profile. There is no published specification and no registered media type.
You do not need this section to fix anything. It is here because the gaps in it explain why so few tools read OLK files properly.
| Property | Value |
|---|---|
| Published specification | None. Microsoft has never documented these files |
| Media type | None registered with IANA |
| Encoding | Binary, with UTF-16 and UTF-8 text inside |
| Filenames | UUIDs, assigned when the item is saved |
| Index | Outlook.sqlite, in the profile root |
| Source files | Standard MIME, behind a binary prefix |
| Attachment files | MIME with the payload in Base64 |
| Folder fan-out | 256 numbered subfolders, 0 to 255 |
That binary prefix in front of every MsgSource file is the one detail worth knowing if you are writing a parser. Strip it and what remains is an ordinary message that any mail library reads.
That prefix is why a MsgSource file renamed to .eml often opens with one line of garbage above the headers.
Parsers that read one OLK file at a time recover messages. It cannot recover a mailbox, because folder names, read state and the message-to-attachment links all live in the database.
The open-source olk15-export tool documents its join, and says plainly that three files from one email carry unrelated identifiers with no filename pairing.
Outlook for Mac 2011 is 32-bit, which is why it stopped running. Microsoft's support note puts it in one line: Apple stopped supporting 32-bit applications. macOS 10.15 Catalina arrived in October 2019, and Outlook for Mac 2011 would not launch.
The end of support date was 10 October 2017, so the olk14 files on a lot of old drives now outlive every program that could read them.
Two questions sort out most of what is left. Answer them in order.
Names ending in olk14 or olk15 are Mac files, and section 4 says which to keep. A folder called Olk is Windows, and section 8 covers it.
Open it in TextEdit. Mail headers mean a MsgSource file worth recovering. Binary noise means a Message file, and section 5 explains why chasing it is wasted effort.
Landed here from a rebuilt Outlook on Windows? Our new Outlook error guides cover that client, including the something went wrong screen and the 0/0GB storage reading.
Trying to get the mail into Windows Outlook? The PST file format is what it writes. The OST file format is what a connected account builds, and Outlook PST error fixes cover export failures.
Came here looking for an address book? The OAB file format is Outlook's address book, and Thunderbird address book files cover the other side. Moved to Thunderbird instead? The MBOX file format is what it writes.
New to all of this? What a file format is covers the ground underneath, and our file format guides list the rest.
Found something here that does not match your Mac? Send us the exact wording on screen and we will check it against Microsoft's own documentation.