What is a PST File And What Outlook Stores in It

A PST file is a database rather than a mail file. Microsoft Outlook writes one to hold email messages, calendar events, contacts and a good deal besides. Inside it keeps an allocation map, two indexes and a numbered entry for every object. That is why PST files hold far more than messages and why they never shrink just because you deleted something. Almost every surprise they produce traces back to that one fact.

The .pst file extension is the part everybody recognises. Outlook calls the same thing an Outlook Data File, Windows shows it as a PST and the specification calls it a Personal Storage Table. Older Microsoft documentation calls the same thing a Personal Folders file or a Post Office File. All of those are one format. Two products carried it before Outlook existed. Microsoft Exchange Client and Windows Messaging both used it, which makes the format older than the program most people associate it with. None of those names conveys the file type you are dealing with. Opening one is closer to attaching a database than to opening a document, which is why nothing outside a mail program will show you anything useful. There is a wider guide to every email file format if you are not yet certain this is the one you have.

What a PST File Is Used For

Before the mechanism, the purpose. People end up with one of these for a small number of reasons and each explains a different thing the format does.

The portability is the point and also the problem. One file carries a whole mailbox to another machine with no special skill needed. That same property is why organisations lose track of them. It is also the difference between a PST file and an OST file, which stays tied to the mailbox that created it.

The PST File Format And Its Three Layers

Microsoft publishes the structure as MS-PST. That document splits it into three layers stacked on each other. Each layer makes sense only once the layer below does, which is why writing a PST reader is serious work rather than a weekend project.

Two details from that bottom layer explain most PST behaviour. Every object carries a node identifier. PST files also keep their own map of which blocks are in use. Delete a message and the messaging layer drops it from its folder, while those blocks are simply marked free in that map. On disk not a byte moves.

Everything PST Files Hold Besides Mail

Ask what PST files hold and the usual answer is emails and maybe attachments. The format was built for the whole of Outlook, so the real list is longer. The extra items are exactly the ones that vanish quietly during a conversion.

Every one of those extra fields is a MAPI property, the same kind an MSG carries for a single item. MAPI is the interface layer Outlook sits on. PST files are MAPI stores, so they have somewhere to put things no message format defines. Everything from Categories downward is configuration rather than content. It describes how Outlook should behave with your mail rather than the mail itself. No interchange format has anywhere to put that. That is not a flaw in the converter. It is the reason the PST existed in the first place.

Search folders are not folders. Search folders hold no messages. They store a query Outlook runs each time you click. Copy a PST file and search folders come with it. Convert one and they vanish, because nothing is there to convert. Every message they appeared to contain is still there, sitting in whatever real folder it always lived in.

What a PST File Does Not Hold

The gaps matter as much as the contents, particularly for anyone handed a PST file and told it is a complete backup of somebody's mailbox.

Which Outlook Accounts Produce a PST File

Whether Microsoft Outlook writes you a PST file at all depends on the account type, which is why some people have one sitting in their Documents folder and others have never seen the extension.

So most PST files in circulation were made on purpose rather than by default. Somebody exported a mailbox or set up a POP account long enough ago that it was still the normal thing to do. The exception is AutoArchive, which moves old items into a PST on a schedule and is the reason plenty of people have one they never knowingly created.

Reasons to make one have not changed since the format arrived. PST files sit on your own disk, so they are not counted against any mailbox quota the server enforces. That single property is why administrators once encouraged them. It is also why so many organisations now hold thousands of PST files scattered across laptops with no idea what is in any of them.

An exported PST is invisible to the server. Anything moved into one leaves the mailbox, so server side search, retention policy and legal hold stop covering it. For a personal archive that is the point. For a regulated organisation it is the reason PST sprawl is treated as a compliance problem rather than a storage one, since an eDiscovery request cannot reach what the server cannot see.

Where PST Files Are Stored in Outlook

Two answers, depending on which kind you have.

That second folder is hidden by default, which is why plenty of people never find the file they are looking for.

Do not hunt through folders. Open Outlook and go to File. Then Account Settings, then Account Settings again, then the Data Files tab. Every PST file the profile knows about is listed there with its full path. Open File Location takes you straight to it. One machine can hold several that Outlook has long since forgotten.

The new Outlook for Windows treats them differently. Support is limited rather than absent. A PST file can be opened, searched and copied from. Full editing is not available in every situation. Anybody relying on one as a working archive should check what the version in front of them actually allows before moving anything.

PST File Size Limit And Why It Never Shrinks

Delete a year of mail, check the size, find it unchanged. The answer is the allocation map. Deleting marks space as reusable inside PST files. It does not return that space to the disk.

Compacting rewrites without those free blocks. Outlook does it slowly in the background when idle. It can also be triggered on demand through the data file settings. On a file that has had a large clear out it can recover a great deal. Nothing else will.

Size limits count bytes on disk, not messages. Microsoft documents a maximum enforced through the registry, measured against bytes on disk. The ceiling depends on which variant you have, since the older ANSI format stops far short of the Unicode one that replaced it. So a mailbox well under the limit can still hit it, purely because deleted items and indexes have not been compacted away. Clearing out mail without compacting afterwards fixes nothing.

What PST Passwords And Encryption Actually Do

Outlook offers a password when you create a PST file. That word implies locked contents. They are not locked.

Stored instead is a CRC-32 checksum of your password text, in a property called PidTagPstPassword. Outlook checksums whatever you type and compares. The specification is blunt about what that achieves, saying the password is implemented at the interface level only and is not used to secure the PST data in any way.

read PidTagPstPassword
prompt the user
CRC-32 what they typed
compare the two values
match, so open the store

read the header
walk the node B-tree
read the data blocks

Outlook also offers an encryption setting at creation, which sounds like the real thing and is not. Two options exist beyond none. Both use keyless ciphers.

Neither takes a key, so neither can be a secret. Microsoft states the position in the specification itself, that the strength of the encoded data blocks provides no additional security beyond data obfuscation.

Only your data is obfuscated. The structure around it is not. The specification notes that headers, allocation pages and B-tree pages are all written without any encoding at all. So even the weak layer covers message content only. Anyone holding one can map its entire shape without decoding a single byte.

The name is a historical accident. A Microsoft engineer has written about where compressible encryption came from. Compressing a PST made its contents unreadable in a text editor. People equated that with encryption. The label stuck. The word would not survive a review now. Confidential mail needs real encryption applied separately, whether that is BitLocker on the volume or an encrypted container holding the file.

What This Means for Anyone Handling a PST File

Damage has its own tool, already installed. Outlook ships with the Inbox Repair Tool, scanpst.exe, which repairs a corrupt PST file by rebuilding its indexes when the file will not open. It works on structure rather than content, so it recovers files more often than it recovers everything in them. Copy first, then run it.

A disconnected PST file does not announce itself. Close one in Outlook or lose the profile that referenced it. It stays on disk with nothing pointing at it. Nothing warns anybody. Orphaned PST files are how organisations end up holding years of email that no backup covers and no search reaches, discovered only when somebody goes looking for a specific message.

Four rules follow from PST files being databases rather than mail files. Each one is the opposite of what a mail file would let you get away with.

1. Never put one on a network share. Microsoft has documented this as unsupported since Exchange 4.0. PST files are read and written with file access commands meant for a local disk, so across a network every operation slows down. Writes take roughly four times as long as reads. A connection dropping mid write can corrupt one outright. Copy to a local disk first. The single exception Microsoft documents is Outlook running remotely on a Remote Desktop Session Host, where file and program sit on the same machine anyway.
2. Close Outlook before copying one. A copy taken mid write can capture a half finished index update. It often opens and looks fine, then fails months later when you reach the part that was being written.
3. Compact after a clear out, not before. Deleting alone changes the size on disk by nothing at all. Until a compact runs, every limit still sees the old figure.
4. Do not rely on the password. Treat PST files as readable by anyone holding one, because they are.
5. Expect configuration to be lost in conversion. Rules, views, categories and search folders have no equivalent anywhere else. Write down what matters before you move anything.

If the job is reading what is inside rather than moving it, a browser based viewer parses the format directly and never needs Outlook or a mail profile. Where the mail is going somewhere else entirely, our mail converters handle the route. The survives column earlier sets out what to expect on arrival.