PST to Office 365 Migration — Every Method Compared (Import Service, Drive Shipping, Tools)

Somewhere in your organization there are PST files — on laptops, file shares, and a USB drive in a drawer. Getting them into Office 365 mailboxes (or better, archives) is a solved problem with four proven methods. Pick by volume:

Method 1 — Import Service network upload (the free workhorse)

1. Collect PSTs to one staging share. Inventory: file → target mailbox → target folder (mailbox or archive).
2. Purview portal → Data lifecycle management → Import. Create an import job; Microsoft issues an SAS URL for a staging Azure blob.
3. Upload with AzCopy: azcopy copy "\\share\psts" "<SAS-URL>" --recursive
4. Build the mapping CSV (Workload=Exchange, FilePath, Name, Mailbox, TargetRootFolder, IsArchive).
5. Validate → import. Optionally filter by age during import (e.g. only items older than 2 years → archive).

Method 2 — Drive shipping

Same pipeline, but you BitLocker-encrypt SATA/SSD drives, ship them to Microsoft, and they load the blob for $2/GB. Worth it above ~1 TB of PSTs or when your uplink makes network upload a month-long project.

Method 3 — Outlook import (don't scale this)

File → Open & Export → Import/Export. Fine for the CEO's one archive file. As a fleet strategy it means touching every machine, no logging, and Outlook throttling uploads through the client. Above ~10 PSTs, use the Import Service.

OST, OLM, EML, MBOX — the conversion cousins

• OST: an offline cache, not an archive. If the account still works anywhere, open it and export a PST properly. Orphaned OSTs need a converter tool — expect imperfect fidelity.
• OLM (Outlook for Mac): no native path — converter tool to PST, then any method above.
• EML/MBOX (Thunderbird, Apple Mail, exports): either IMAP-drag into a live mailbox (free, slow) or convert to PST. For whole-org moves from IMAP platforms, skip conversion — migrate server-side: see IMAP to Microsoft 365 migration.

FAQ

Is Microsoft's PST Import Service really free?

Network upload: yes, free with the required licenses. Drive shipping costs $2/GB plus the drives and postage.

How long does a PST import take?

Upload at your bandwidth, then Microsoft's processing typically runs 24–72 hours per job regardless of nagging. Plan a week end-to-end for anything sizable.

Can I import PSTs directly into user archives?

Yes — set IsArchive=TRUE in the mapping CSV. Enable each user's archive mailbox first.