How to export a Microsoft 365 mailbox to PST (and when not to)

Sometimes you genuinely need a mailbox in a file. A lawyer asks for a copy of a leaver's correspondence, you're moving a company off Microsoft 365, or a tenant is being shut down and someone wants the mail on a disk first. For those jobs, exporting a Microsoft 365 mailbox to a PST file is a legitimate, free, built-in answer — and this post walks through both ways to do it. It also covers, honestly, what a PST export can't do, because "we exported it to PST" is doing a lot of quiet work in many companies' retention strategies.

First: is a one-off export actually what you need?

A PST export is a snapshot. It captures whatever is in the mailbox at the moment you run it, and it never updates afterwards. That makes it the right tool when the job is finite and one-directional:

• Handing mail to a third party — a lawyer, auditor, or insurer wants a copy of specific correspondence.
• Leaving Microsoft 365 — you're migrating to another platform and need the mail out.
• Decommissioning a tenant — the company is closing or merging and the mailboxes are going away.

If, on the other hand, the sentence in your head is "we export leavers' mailboxes to PST so we have an archive", stop before step one — that plan has structural problems no export procedure fixes, and we'll get to them below. (Spoiler: PST files are not an archive.)

Option 1: export from Outlook

The simplest route, no admin roles required — if you can open the mailbox in classic Outlook for Windows, you can export it. (The new Outlook for Windows and Outlook on the web don't offer PST export; Outlook for Mac exports to .olm, a different format. This is a classic-Outlook-on-Windows job.)

1. Open the mailbox in classic Outlook. For someone else's mailbox, an admin first grants you full access permission, then you add it to your Outlook profile and let it sync.
2. Go to File → Open & Export → Import/Export.
3. Choose Export to a file, then Outlook Data File (.pst).
4. Select the mailbox at the top level and tick Include subfolders.
5. Pick a destination path, decide how duplicates are handled, and (optionally) set a password. Then wait — large mailboxes take hours.

Option 2: export via Microsoft Purview content search

The admin-side route: Microsoft Purview's eDiscovery tooling can search any mailbox in the tenant and export the results as PST — no Outlook profile, no full-access grants, and it works for mailboxes nobody wants to mount locally. A caveat before the steps: Microsoft reworks this portal regularly, so button labels drift — but the shape of the flow has been stable for years.

1. Sign in to the Microsoft Purview portal with an account that holds an eDiscovery Manager role. Global admin alone isn't enough by default — the role has to be assigned.
2. Open eDiscovery and create a case (in the current experience, even simple content searches live inside a case).
3. Create a search: pick the target mailbox as the data source. Leave the query empty to capture the entire mailbox, or add conditions (date range, sender, keywords) to narrow it.
4. Run the search and review the estimate — item count and size tell you whether the scope looks right.
5. Choose Export, select the PST output format, and once the export job finishes, download the results from the export tab.

Things worth knowing before you rely on it: exports are split into multiple PST files beyond a size threshold (10 GB per file by default), export jobs are throttled and can take many hours for big mailboxes, and the download links expire after a while — this is built as an eDiscovery tool for legal requests, not as a bulk-backup pipeline. It also only exports what exists right now (plus anything held in Recoverable Items if the mailbox is under a hold) — mail deleted before you searched is gone.

Which option for which situation

One timing note for offboarding: both options need the mailbox to still exist. Once a leaver's license is removed, a 30-day countdown starts — export before that, or understand what happens to a Microsoft 365 mailbox when an employee leaves before you touch the account.

What a PST export can't do

Both procedures above work. The problems only start if the PST is expected to behave like a retention system afterwards:

• It's frozen. Mail that arrives after the export isn't in it. An "archive" that requires someone to remember to re-export is a calendar reminder, not a record.
• It's incomplete by design. Anything deleted before export day was never captured. The file preserves the mailbox the user chose to leave behind.
• It's editable. Anyone who opens a PST in Outlook can delete or move messages. For anything that hinges on proving mail is unaltered, that disqualifies the format.
• It's fragile. Large PSTs are notorious for corruption, and a file on a share is one cleanup, one ransomware incident, or one dead USB drive away from gone.
• It doesn't search. Finding one email across a folder of PSTs means mounting them in Outlook one by one and waiting for indexing. In practice, nobody does.

The full argument, with what each failure looks like in real life, is here: PST files are not an archive.

If the real goal is retention, not a file

Most PST-export routines exist because a business needs to keep email — for retention obligations, disputes, or continuity after people leave. That job wants a system, not a snapshot: every message captured automatically as mail flows, stored so nobody can quietly alter it, and searchable years later without mounting anything. Inside Microsoft 365 that conversation leads to Purview retention policies versus a dedicated archive — either beats a folder of PSTs.

If a dedicated archive is the answer, mailvault365 replaces the export routine entirely for Microsoft 365: connect your tenant once, and every mailbox — including shared mailboxes and departed employees' — is archived continuously into tamper-evident storage with full-text search across all of it. And when someone does need mail as files, any message downloads as a standard .eml — an open format that any mail client reads, with no sync sliders, no corruption folklore, and no eleven-PST scavenger hunts.

Export a PST when you need to hand mail over. For everything you need to keep, retire the ritual.