Back Up Outlook.com and Microsoft 365 Without PST Files

You can back up Outlook.com, Hotmail or Microsoft 365 without ever touching a PST file. All it takes is a program that connects to your account over IMAP using Microsoft sign-in and saves your mail to your own drive in an open format like mbox. PST is still Microsoft’s official export route, but it requires desktop Outlook, it’s a manual export that’s out of date the next day, and Outlook is about the only thing that reads it well. For a backup that keeps itself current, IMAP is the better fit.

What Microsoft covers on its own

First, it helps to know how much room you have if you delete something:

• Outlook.com and Hotmail: Deleted Items empties itself after 30 days, and items removed from that folder can be recovered for another 30.
• Microsoft 365 for business: permanently deleted items are kept for 14 days by default, and an admin can raise that to 30.

After that, the message is gone. And that’s before account closures for inactivity or hijacked logins, which we covered in Your Email Is in the Cloud. That Isn’t a Backup.

The traditional route: exporting to PST

A PST (Personal Storage Table) is Outlook’s data file. It holds mail, contacts and calendar in a single file, and it’s what Microsoft points you to for a copy of your mailbox.

According to Microsoft’s help page, exporting an Outlook.com mailbox requires desktop Outlook: new or classic Outlook for Windows (legacy Outlook for Mac exports to a different format, .olm). You add the account, run the export, and the file contains your mail, contacts and calendar up to the day you create it.

It works, but as a backup it has some real drawbacks:

It’s manual and one-off. Each export is a snapshot of that day. Anything that arrives or gets deleted afterward isn’t in the PST until you export again, and very few people do that on a regular schedule.

It has a size cap. In Outlook 2010 and later, a PST stops at 50 GB by default. You can raise it with a registry edit, but Microsoft warns that performance may suffer.

It isn’t meant to live on a network share. Microsoft says PST files aren’t meant to be a long-term, continuous-use method of storing messages in an enterprise environment over a network, and that using them that way can corrupt the file if the connection drops. If the plan was to park PSTs on the office NAS, think again.

Opening it depends on Outlook. Microsoft does publish the format’s specification, but in practice Outlook is what opens it. Even the new Outlook for Windows needs classic Outlook installed alongside it, plus an account with a Microsoft 365 subscription, to open a PST, and it doesn’t work on the ARM version.

None of this makes PST bad. For a move, or for saving a mailbox before you close an account, it does the job. As a backup that stays up to date, it falls short.

The alternative: IMAP with Microsoft sign-in

Outlook.com and Microsoft 365 both support IMAP, the same protocol almost every email app uses. One important detail: Microsoft is retiring Basic authentication (plain username and password) for Outlook.com, so apps need to use modern Microsoft sign-in (OAuth). In practice, when you add the account a Microsoft page opens, you sign in there and grant permission, and the app doesn’t need to store your password.

With that in place, a backup program can download the whole mailbox once, then fetch only what’s new, automatically.

Step by step

1. On a work account, check with your admin. In Microsoft 365, admins can turn off IMAP for users. If they have, a program can’t connect that way.
2. Pick where the backup goes. An external drive is the simplest option. Check how much space your mailbox uses in Outlook’s settings and leave some headroom.
3. Add the account in your backup program using the Outlook.com or Microsoft 365 option, and sign in with Microsoft when prompted. If you use two-step verification, you’ll complete it in that same window.
4. Run the first backup. It’s the slowest, since it downloads everything. Later runs are much quicker.
5. Check the result. Search for a specific old email and open one of the saved files in another program to make sure it’s all there.

PST vs. IMAP backup

	PST export	IMAP backup to mbox
What you need	Desktop Outlook	A backup app
How often	Manual, whenever you remember	Automatic
Mail deleted afterward	Missing until the next export	Kept in the backup
Size cap	50 GB by default	Whatever fits on the drive
On a NAS	Microsoft advises against it	Fine, if the app supports it
What opens it	Mainly Outlook	Thunderbird, Apple Mail, mbox viewers
Contacts and calendar	Included	No (email only)

Microsoft details checked in October 2026 against the help pages linked above.

That last row deserves a straight answer: if you need contacts and calendar too, PST includes them and an IMAP backup doesn’t. You can combine both: automatic backup for email, plus an occasional export of contacts and calendar.

How Arca does it

Arca Mail Backup connects to Outlook.com, Hotmail and Microsoft 365 over IMAP with Microsoft sign-in and saves your mail to the drive you choose as standard mbox files, one per folder and month, with no PST involved. Your sign-in stays in the Windows Credential Manager, and there’s no server of ours between your computer and Microsoft.

Anything you delete in Outlook stays in the backup, marked as deleted, and you can search for it and restore it to the same account in a new folder. Arca backs up email only, not contacts or calendars. For organizations that block IMAP, a Microsoft Graph connection is coming soon to Pro.

The free plan covers one account backed up to an internal or external drive. Pro adds more accounts and lets you save to a NAS or S3-compatible storage. It’s all on the pricing page and in the FAQ.

Arca for Windows is in the Microsoft Store. For Mac, it’s in the Mac App Store.