Outlook .pst vs .ost files explained — and when to convert one to the other

A guy in Reno emailed me last September in full panic mode. His old Dell had finally died — the fan had been screaming for weeks, he ignored it, the motherboard cooked itself on a Tuesday afternoon. He'd pulled the SSD, put it in an enclosure, and copied his entire Outlook folder to the new ThinkPad. Then he double-clicked the file and got the error he didn't want: "The file C:\Users\...\outlook.ost is not an Outlook data file (.ost)."

He thought he'd lost twelve years of email.

He hadn't. But he also hadn't actually copied any email. That's the thing nobody explains about these two file types — they look the same, they sit in the same folder, they end in three letters that differ by one. And they do completely different jobs.

What each file actually is

A PST is an archive. It's portable. Self-contained. You can email it to yourself (well, you can't, it's too big — but conceptually). Drop it on a USB stick, plug it into another machine, open Outlook on that machine, and the data shows up. It's the closest thing Outlook has to a "save as" file.

An OST is a sync cache. It's a local copy of what's already sitting on Microsoft's servers. The "O" stands for offline — it exists so you can read mail on a plane. Nothing more. The mail isn't in the OST in any meaningful sense. The mail's in the mailbox. The OST is a mirror.

This distinction matters because of what happens when you move the file. Move a PST to a new machine and the data moves with it. Move an OST and you're moving an empty shell that needs the original Microsoft account to mean anything. And the new Outlook installation won't even recognize it as the same OST — it'll build a fresh one from scratch the moment you sign in.

So my Reno friend was fine. His mail was on Microsoft's servers. He just needed to sign in.

Where these files live

If you've never gone looking, the path is buried. On Windows 10 and 11 it's:

C:\Users\<yourname>\AppData\Local\Microsoft\Outlook\

AppData is hidden by default. Type the path into File Explorer's address bar and it'll take you there. You'll see one .ost per Microsoft 365 / Exchange / Outlook.com account, and (if you've made or imported any) some .pst files alongside.

PST files used to default to Documents\Outlook Files in older versions. If you've upgraded over the years you might have PSTs scattered across both locations. I've seen machines with PSTs in three different places — original Documents folder, AppData, and a manually moved copy on D:\. All of them get loaded into Outlook on startup if they're still listed in Account Settings → Data Files. Worth a look if Outlook starts slow.

"But I want to take my mail with me"

Right. So here's the move.

If your mail lives on Microsoft 365, Exchange, or Outlook.com — you don't need to do anything with the OST at all. Sign in on the new machine. Outlook builds a fresh OST. Mail syncs down. Done. Depending on mailbox size and connection it's anywhere from ten minutes to overnight.

If your mail lives on a POP3 account where everything's local, or you specifically want a backup file you can store on a separate drive, that's where PST export comes in. In Outlook desktop go to File → Open & Export → Import/Export → Export to a file → Outlook Data File (.pst). Pick the folder (or the whole mailbox), pick a destination, and you get a .pst you can do whatever you want with.

A few quirks here. The export runs on the foreground thread, so Outlook locks up while it's working. On a 30 GB mailbox over a slow Exchange connection that can be an hour. And the export reads from the OST cache, not the server — so if your OST hasn't fully synced down yet, the export will be incomplete. Let it sit overnight before exporting if you've just installed.

The OST-to-PST tool industry

Search "convert OST to PST" and you'll get fifteen ads for tools that promise to extract your mail from a dead OST file. Most of them are useless. A few of them are scams. And here's the part that nobody selling these tools wants to admit: in 95% of cases you don't need them, because the mail is in the mailbox, and you can just sign in.

The 5% where these tools do something useful — that's specifically when you've got an OST from an Exchange account that no longer exists. Account got deleted. Server got decommissioned. Whatever. The OST is the only remaining copy of that mail. In that scenario, yeah, you need a tool to read the OST format and dump it to PST. Stellar makes a real one. Kernel makes one that works. Most of the others are wrappers around the same handful of libraries.

But before you spend $79 on a license, ask: is the original mailbox really gone? Because if the answer is no — if it's just that you don't have the password handy, or the IT guy at your old job won't help, or you're not sure — then chasing the OST conversion is the wrong path. Get back into the mailbox.

File size limits

PSTs and OSTs both default to a 50 GB max in current Outlook versions. You can push that to 100 GB by editing the registry — it's the MaxLargeFileSize value under HKEY_CURRENT_USER\\Software\\Microsoft\\Office\\16.0\\Outlook\\PST. Set it as a DWORD in MB, so 102400 for 100 GB.

I've never seen a real reason to go above 50 GB. Once a data file gets that big, Outlook starts behaving weirdly — search indexing slows to a crawl, folders take seconds to switch, the whole app feels like it's wading through molasses. If you're at 50 GB you have an archiving problem, not a file size problem.

The other thing that gets people: the limit is on the file, not the mailbox. Your Microsoft 365 mailbox can be 100 GB on the server. But if your local OST hits 50 GB, sync starts failing silently. You won't get a clear error. Mail just stops appearing. I had a client whose entire 2024 inbox never synced because his OST capped out in December 2023 and he didn't notice until April.

What happens when you delete an OST

Nothing bad. Promise. Close Outlook, delete the .ost file, reopen Outlook. It builds a new one. Sync starts over from scratch.

This is actually a useful troubleshooting move. If Outlook's behaving weird — folders missing, search broken, mail showing up out of order — nuking the OST and letting it rebuild fixes about half of those issues. It's the Outlook equivalent of "have you tried turning it off and on again."

What you do not do is this with a PST. Delete a PST and the data's gone. There's no rebuild. The PST was the data.

AutoArchive and where PSTs come from

A lot of people have PSTs they didn't create on purpose. AutoArchive is the reason. Outlook has a built-in feature that automatically moves old mail out of your main mailbox into a local PST file called archive.pst. It's been on by default in some versions, off in others, and the rules for what gets archived are buried under File → Options → Advanced → AutoArchive Settings.

If you're sitting at a machine wondering "where did this Archives folder come from at the bottom of my folder list" — that's AutoArchive. It made you a PST. The data in there is local to that machine. It will not follow you to a new computer unless you bring the PST with you.

I'd turn AutoArchive off for any modern Microsoft 365 account. The mailbox quota is large enough that you don't need it, and silently moving mail to a local file that doesn't sync is a great way to lose data later.

OneDrive and PSTs — please don't

This one's important and the Microsoft docs on it are buried. Do not put a PST on OneDrive. Don't put it in OneDrive's local sync folder. Don't move it to a synced location. Don't get clever.

OneDrive is built for files that get opened, edited, saved, closed. PSTs stay open the entire time Outlook is running. The sync engine and the PST file format fight each other constantly. You'll get sync conflicts. You'll get corruption. You'll lose mail. Microsoft has explicit guidance against this and they're right for once.

If you want a PST in cloud storage for backup, fine — copy it there manually with Outlook closed, when you actually want to back it up. But don't have the live file sitting in a sync folder.

The TL of it

PST is a portable archive you can move anywhere. OST is a local cache that only means something to its original account. If you're moving machines on a modern mailbox, just sign in and let it sync — don't bother with the OST. If you genuinely need a backup file, export to PST and store it somewhere that isn't OneDrive. And if your OST is acting up, deleting it is safe and often fixes things.

That covers about 90% of the situations I see. The other 10% are the weird ones — orphaned OSTs from defunct accounts, corrupt files, encryption issues. Those are case-by-case. Email me if you hit one I haven't covered.