Microsoft 365 email uses the .pst format for migration.

Microsoft 365 email uses the .pst format for migration. That is the plain answer, and it is the part that matters when a mailbox has to move without losing the pieces that give it shape.

I keep the first rule simple. A migration is not finished when the export bar reaches the end. It is finished when the mail still fits together after the move. That means messages, folders, contacts, calendar items, and tasks need to survive the trip in one piece. The file type used for that job is the Outlook Data File, better known as PST.

PST is not some odd side path. It is the standard file Outlook uses when mail is exported to a file. Microsoft’s own support pages describe exporting mail, contacts, calendar items, and tasks to a PST file from Outlook, and Microsoft also documents importing PST files into Microsoft 365. That is the core of the matter. Microsoft 365 and Outlook use PST as the handoff format when mail has to leave one place and land in another.

That matters for a simple reason. Microsoft 365 is a live mail system. A migration asks it to become a file source for a while. PST is the container that makes that possible. It can hold mailbox data in a form that can be moved, stored, or brought back later. In plain words, it is the shipping box for the mailbox.

The useful detail is that PST is not limited to just email. It can also carry contacts, calendar entries, and tasks when the export includes them. That is why a migration that only checks the Inbox is still not done in any useful sense. Folders below the surface matter too. So do shared items, buried folders, and the odd little note a person forgot was there until it vanished.

There is one quiet trap here. Outlook and Microsoft 365 do not behave exactly the same in every setup. Export options can vary by Outlook version, mailbox type, and admin rules. Microsoft also changes the details over time. So the broad fact stays steady, but the exact path can shift. The file format is still PST. The buttons and menus may not stay still.

I care about that limit because it is where many bad assumptions start. People see “export” and think “backup.” Those are not the same thing. A PST export is a file copy of mailbox content at a point in time. It is useful. It is not magic. If a mailbox is damaged, the export may carry that damage with it. If folders are missing at export time, they are not likely to appear by wishful thinking later. Email systems do not reward optimism.

The other point worth holding onto is that PST is a migration format, but not always the final home. A person may export from Microsoft 365 into PST first, then import that PST elsewhere, or keep it as an archive. That is normal. It is also why the file matters so much. Once the move depends on a file, that file becomes the whole job. If the file is incomplete, the migration is incomplete.

That is the part I return to when the topic gets messy. The real question is not whether Microsoft 365 can use PST. It can. The real question is whether the exported file contains everything that needs to survive. A mailbox is easy to count badly. The Inbox gets attention. The old folder tree does not. Contacts get skipped. Calendar data gets treated like decoration. Then someone opens the result and starts hunting for what went missing. That is where the work really begins.

So the answer stays direct. Microsoft 365 email uses the .pst format for migration, and that format matters because it carries mailbox data in a portable file. The useful habit is to treat the export as a full mailbox copy, not a quick save. If the mail, folders, and contacts are all there, the move has a chance to count as done.

That is the kind of plain problem The Migration Brief tries to keep in view: one practical fix, one file format explained, and one safer way to move your mail.