Backing up Outlook mailboxes and other large files
Outlook keeps its mailbox open all day. How GBM AI backs it up anyway, how a continuous job saves only the parts of a large file that changed, and how to get a mailbox back.
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An Outlook mailbox is one large file, often several gigabytes, that Outlook keeps open while it runs and changes a little with every message. Two things matter when you back it up: reading it while Outlook has it open, and not storing gigabytes again for every small change. GBM AI does both.
Where Outlook keeps your mail#
.pstfiles (Outlook data files) hold mail from POP accounts, your archives, and anything you saved to a data file. This mail may exist nowhere else: back these up..ostfiles are Outlook's offline copy of an Exchange, Microsoft 365, Outlook.com or IMAP mailbox. The mail itself stays on the server, and Outlook can download it again into a new.ost.
Outlook keeps them in Documents\Outlook Files or in %LOCALAPPDATA%\Microsoft\Outlook. You do not have to find them: the Email template, or the Outlook data item on the wizard's What step, picks up both places. See Choosing what to back up.
Backing it up while Outlook is open#
Outlook locks its files while it runs, so they cannot simply be copied. GBM AI asks Windows for a snapshot of the drive and reads the mailbox from that, in a moment, without closing Outlook. Windows allows snapshots only with administrator rights, so one of these is needed:
- Service mode (Pro and Server): snapshots on every run, with nothing to approve. See Running backups with nobody signed in.
- Restarting the agent as administrator, which lasts until Windows restarts or you sign out.
Without either, a mailbox Outlook has open is skipped and listed on the run report as In use by another program. Everything else is backed up as usual, and closing Outlook before the backup works too. See Needs administrator: open files, VSS and three ways round it.
Scheduled jobs save the whole file when it changed#
A scheduled job (daily, weekly and the like) saves a file again whole whenever it changed. Outlook changes its mailbox every time it runs, so after a day of mail each backup stores the mailbox again, compressed. A 3 GB mailbox backed up every night adds about one copy of it a night, less what compression saves, and retention removes the old ones as they expire.
That is fine for many people. To save a mailbox all day without storing it again each time, use a continuous job.
Continuous jobs save only the parts that changed#
In a continuous job (Pro and Server), a file of 16 MB or more is saved in blocks. GBM AI cuts the file into pieces, recognises the pieces it has already stored, and saves only the new ones. Each version of the file is a list of its pieces, so every version restores whole, while the backup keeps each piece only once.
Measured with a 3 GB Outlook mailbox:
| What changed in the mailbox | What the backup stored |
|---|---|
| Small changes scattered through it, as reading and flagging messages make | 1.4 MB |
| 50 MB of new mail | 51.7 MB |
| Something inserted early on, which moved everything after it | 1.8 MB |
| The end of the file rewritten | 315 MB |
When a large file is saved:
- After it has not changed for 2 minutes, or as soon as its program closes it, so a file still being written is never saved half-done.
- A file in use, such as the mailbox while Outlook runs, is saved from a snapshot at most every 30 minutes (you can choose another interval on the Schedule step) and as soon as Outlook closes it. This needs service mode; without it, the mailbox is saved when Outlook closes it.
- A file of 1 GB or more is read in full each time to find what changed, so it is saved at most as often as files in use.
Each piece is compressed, and encrypted when the job is. It works the same to a drive, a network share or an S3 bucket. See Continuous protection and Protect your files continuously, step by step.
Getting a mailbox back#
Restore it like any other file: pick the moment (a continuous job's timeline lets you go back to the minute), tick the .pst, and choose where it goes. See Restore a file or folder.
- Close Outlook first if you restore over the mailbox Outlook uses: a file Outlook has open cannot be replaced.
- Or restore it to another folder and open it in Outlook with File ▸ Open & Export ▸ Open Outlook Data File. You can then copy back only the messages you need, and leave your current mailbox as it is.
A mailbox saved in blocks comes back whole, and every piece is checked as it is read. Backups that hold files saved in blocks need GBM AI 10.5 or later to restore; older versions refuse them rather than read them wrongly.
Other large files#
The same applies to any file of 16 MB or more in a continuous job: accounting files and other databases, large Word, Excel and PowerPoint files, disk images, video projects. A program that keeps its file open all day needs service mode for the file to be saved while open, as Outlook does.