For a small business, the mailbox is one of the most valuable archives there is: it holds quotes, contracts, order confirmations and the entire history of every client and supplier relationship. Yet almost no one keeps a copy that is independent of the cloud. This guide shows how to create a complete backup of an email account in Outlook 365 for Windows and how to bring it back when you need it, working with the classic Outlook desktop app and the PST data file. The process is straightforward, but a few checks up front are what keep your backup from coming out half-empty.
Why keep a local copy of your mail
Relying on the online mailbox alone is fine until the day access disappears. That day can arrive in several ways: a lapsed Microsoft 365 licence, an account locked for security reasons, an accidental deletion, or malware that encrypts your data. When it happens, having a PST file on your own disk is the difference between recovering in minutes and losing months of correspondence.
- Vendor independence: a PST opens on any PC running Outlook, even without the original mailbox.
- Smoother migrations: change domain or provider and your archive travels with you.
- Archiving: you can slim down the online mailbox while keeping the full history safe.
Check the free space on your disk
Before touching Outlook, work out how much room you need. The backup file will be roughly the total size of the mailbox, so the destination drive must have free space greater than that figure. To check:
- Open File Explorer and click This PC in the left-hand column.
- Under each drive (usually the C: disk) you will see a bar showing the available space in GB.
- Estimate the mailbox size: in Outlook go to File > Tools > Mailbox Cleanup > View Mailbox Size, or look at the data file size in the account settings.
If the mailbox weighs, say, 15 GB, make sure you have at least 20 GB free at the destination so you are working with some headroom.
What to do if there isn’t enough space
Do not start the export until the disk has enough capacity: a backup interrupted halfway through is useless. You have two options.
- Free up space on the internal disk: run Windows Disk Cleanup, empty the Recycle Bin, delete temporary files and downloads you no longer need, uninstall unused programs, or move heavy folders such as photos and videos elsewhere.
- Save the backup to an external drive: a roomy USB stick or an external disk works fine, as long as the free space exceeds the mailbox size.
Whichever destination you choose, recheck the available space with the method above before going on.
Set Outlook to download all your mail
By default, with Microsoft 365 and Exchange accounts Outlook keeps only the most recent messages locally, often just the past year. Export in that state and the PST will contain only part of the mailbox. So you need to tell Outlook to download the entire history:
- Go to File > Account Settings > Account Settings.
- On the Email tab, double-click the account in question.
- Find the Download email for the past: slider and move it to All.
- Confirm with Next and then Finish.
This option lives in the classic Outlook desktop app; the new Outlook for Windows handles synchronisation differently and does not offer PST export.
Restart Outlook and wait for the sync
Once you have changed the setting, close Outlook completely and reopen it: only then does the app begin downloading the full history locally. Depending on the size of the mailbox and the speed of your connection, this can take anywhere from a few minutes to several hours.
- Keep the computer on and connected to the network for the whole process.
- Watch the status at the bottom of the window: when it reads All folders are up to date, the sync is complete.
- Do not start the backup before that point, or you will get a partial copy.
If the mailbox is very large, it is worth letting Outlook work overnight and carrying on the next day.
Export your mail to a PST file
With everything stored locally and enough space in place, you can create the actual backup. The procedure uses Outlook’s export wizard:
- Open File > Open & Export > Import/Export.
- Choose Export to a file, then Outlook Data File (.pst).
- Select the top-level folder of the account and tick Include subfolders to copy everything.
- Point the destination to the drive you verified earlier and, if you wish, set a password.
- Start the export and wait for it to finish.
When it is done, check that the PST file exists and that its size matches the mailbox: a file much smaller than expected points to an incomplete export.
Restore a PST file into Outlook
When you need to bring the mail back, on a new PC or into the same account, the PST is restored with the same wizard run in reverse:
- Go to File > Open & Export > Import/Export.
- Choose Import from another program or file, then Outlook Data File (.pst).
- Select the backup file and decide how to handle any duplicates.
- Choose whether to import into the same account or into a separate folder, then confirm.
If you only need to browse the contents without merging them into the mailbox, you can open the PST read-only via File > Open & Export > Open Outlook Data File: it will appear as an extra archive in the folder list.
A special thank-you to our follower, friend and former colleague Paula de Carlo for suggesting this topic.
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