Archive folder, Online Archive, AutoArchive or PST: which Outlook archive is which?

In Outlook, “archive” can mean four different things:

• the Archive folder, where the Archive button sends mail, which is just another folder in your mailbox;
• the Online Archive, called In-Place Archive in new Outlook and Outlook on the web, which is a second mailbox on the server for work and school accounts;
• AutoArchive, an older classic Outlook feature that moves old mail out of your mailbox into a PST file on your computer;
• a PST file (Outlook Data File), which holds mail on your computer, whether AutoArchive made it or you did.

Only the last two move mail off the server, and only the Online Archive and the PST options free space in your mailbox. Mixing them up is the most common reason people think archived mail has disappeared.

The Archive folder: a tidy Inbox, not more space

The Archive button moves the selected messages from the Inbox to a folder called Archive. Microsoft’s Archive in Outlook article describes it as a way “to move one or more messages to your Archive folder without deleting them,” and is clear about what it does not do: it “doesn’t reduce your mailbox size.”

For Microsoft 365, Exchange and Outlook.com accounts the Archive folder is a built-in folder in the mailbox and “You cannot change the Archive folder location.” For POP and IMAP accounts you can choose which folder is used. Because it is part of the mailbox, the Archive folder syncs to every device, is searched with the rest of the mailbox and is kept on the server like everything else.

Use it to keep the Inbox short. Do not use it to solve a “mailbox full” warning.

The Online Archive: a second mailbox on the server

The Online Archive is a separate archive mailbox attached to your work or school mailbox. Classic Outlook and Outlook for Mac call it Online Archive; new Outlook and Outlook on the web call it In-Place Archive, and Microsoft’s online archive article warns: be “careful not to confuse the Online Archive or In-Place Archive folder with your existing Archive mail folder.”

It does free space: mail in the archive mailbox does not count towards your main mailbox’s quota. Its size depends on the licence. According to Microsoft’s Exchange Online limits (updated 19 October 2026):

The 1.5 TB figure relies on auto-expanding archiving, which an administrator turns on and which adds space as the archive fills. Microsoft announced in October 2026 that the ceiling is rising to 3 TB for E5-class licences, with “the existing 1.5 TB limit” unchanged for everyone else (Microsoft Learn).

Mail can move there on its own

Many people never move anything into their Online Archive and still find old mail there. Exchange Online’s default retention policy, applied automatically to new users, includes a tag called Default 2 years move to archive: items older than 730 days move to the archive mailbox. Microsoft notes the policy only runs on mailboxes larger than 10 MB. Your organisation may have changed the period, or turned the policy off.

So if mail older than about two years is missing from a work mailbox, look in Online Archive or In-Place Archive in the folder list first. In classic Outlook, also remember that search only covers the current scope; searching from your main mailbox does not return archive results unless you widen it.

AutoArchive: the older feature that writes a PST

AutoArchive is a classic Outlook feature that moves (or deletes) items once they reach a certain age. Unlike the two above, it moves mail out of the mailbox into a file on your computer. Microsoft’s AutoArchive settings article gives the default file as C:\Users\YourUserName\Documents\Outlook Files\archive.pst and the default ages as:

• six months for the Inbox, Drafts, Calendar, Tasks, Notes and Journal;
• two months for Sent Items and Deleted Items;
• three months for the Outbox.

Microsoft now describes AutoArchive as “an older Outlook feature” and the Online Archive as “an enterprise-grade replacement for AutoArchive.” Three things are worth knowing:

1. AutoArchive is not available in new Outlook. Microsoft states: “AutoArchive feature is not available in new Outlook for Windows.” Moving to new Outlook does not delete an existing archive.pst, but it stops opening it for you.
2. The archive exists only on that computer. It does not appear on your phone or in Outlook on the web, and it is not backed up by Microsoft. If the PC is replaced or the disk fails, the archive goes with it unless you copied the file.
3. Your organisation may turn it off. Microsoft notes that Archive and AutoArchive “might not be available” when your profile connects to Exchange, where server retention policies usually take over.

PST files: mail that lives in a file

A PST (Outlook Data File) is a file of mail, contacts and calendar on your computer. AutoArchive creates one, but you can also create a PST, export mail to one, import one, or simply open one in Outlook alongside your mailbox. These are different operations: opening a PST shows its folders without copying anything; importing copies its contents into your mailbox; exporting copies mail out of the mailbox into a new PST.

A PST is portable and independent of any account, which is useful for keeping an old mailbox after leaving a job or a provider. The flip side is that it is only as safe as the disk it is on, it can be damaged by being used over a network or a sync folder, and it is not visible on other devices.

New Outlook has gained most PST features during 2025 and 2026, including import and export, but opening a PST still requires classic Outlook to be installed on the same PC, and does not work on Windows on ARM. Our guide to PST files in the new Outlook has the details and dates.

Which one should you use?

• To tidy your Inbox: the Archive button. Nothing leaves the mailbox.
• To free space in a work mailbox: the Online Archive, if your licence includes one. Ask your IT team; it may already be on.
• To keep mail from an account you are leaving, or to hold a copy outside any account: export to a PST, and keep more than one copy of it.
• AutoArchive: only if you are on classic Outlook, have no Online Archive, and understand that the result lives on one computer.

Finding archived mail that has “gone”

1. Look for Archive (the folder) in your mailbox.
2. Look for Online Archive or In-Place Archive below your mailbox in the folder list.
3. Look for Archives in the folder list, the PST that AutoArchive creates. If it is not there, find archive.pst and open it with File › Open & Export › Open Outlook Data File.
4. Search with the scope set to All Mailboxes, which includes open PSTs.

If you have an archive PST but no Outlook to open it in, or new Outlook will not open it, the PST Viewer opens it in your browser without uploading the file, and shows every folder and how many messages it holds. For more reasons mail can seem to disappear, see emails missing from Outlook or a PST.

Sources

• Microsoft Support: Archive in Outlook for Windows, Manage email storage with online archive mailboxes, AutoArchive settings explained, Archive older items automatically, Open and find items in an Outlook Data File (.pst).
• Microsoft Learn: Exchange Online limits (19 October 2026), Learn about auto-expanding archiving (13 October 2026), Default retention policy in Exchange Online (29 May 2026), Release notes for new Outlook.