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Which .pst file is backed up?

Feb 9, 2016 at 12:16 PM
Hi there
I'm using Outlook 2016 Desktop as part of Office 365...
If I perform a manual Backup via the 'Export' feature and select all email folders, I currently end up with a file of 4233 KB.
However, I noticed that the default Outlook Data File is only 2993 KB and wondered why the difference?
And further to that, if I use your Add-In, does it definitely capture the complete snapshot of all my emails?
Thank you in anticipation of some guidance.
Feb 9, 2016 at 9:00 PM
outlook stores different data on different locations, e.g.
  • calendar
  • emails
  • account settings
  • signatures
  • outlook settings
Furthermore emails can be cached in ost files or saved in pst files.
The plugin copies all pst-files to a destination folder.
Additional data can be backed up via script, see

ost-files are used for IMAP or Exchange accounts and are only used for caching purposes, thus no backup usually needed (Except if you store calendar there at IMAP accounts).

Hope this helps,
Kind regards,
Feb 9, 2016 at 9:28 PM
Hi Thomas

Thank you for your reply, although I'm still not much wiser...

Within Outlook, I have 2 accounts set up:

1 x pop3 email account with no Contact or Calendar information and...

1 x Microsoft Account, not used at all for email, but which contains my Calendar, Contacts etc. The associated data for this (I assumed) resides within a .ost data file, not a .pst file.

So I still don't understand why my manually-created .pst file differs in size from the default one.
(I don't need to worry about backing up the Microsoft Account, as this is maintained online).

Having bored you with all that, I guess all I'm asking is whether your Add-On will backup all my pop3 emails successfully?

Feb 10, 2016 at 6:08 AM

if emails, calendar and contacts are stored within two pst-files, it is sufficient to back up these pst-files.
So the backup add-in will save all your emails within the backup.

You can also open the backup pst-files within outlook and convince yourself that all mails are stored there.

Kind regards,
Feb 10, 2016 at 10:09 AM
Hi again Thomas

I installed your add-in and it appears to be fine, thank you.

The one thing I still don't 'get' is why the file size of any manual backup I perform is larger than the automatically-created one using your add-in.

But maybe that's a question for Microsoft, not to hassle you about?
Feb 10, 2016 at 9:52 PM
emails can be stored in different formats, e.g. UTF8 or ISO-Encoding.
Different encodings will result in different sizes.
Different sizes doesnt necessarily mean loss of data.
Details could only tell microsoft.

As mentioned.. you can mount the backup file and verify the number of emails.

Kind regards,