We are getting ready to migrate some email users to Office 365 from a 3rd party email host. They use outlook 2010 to access their email on Exchange server. I know how to export to a .pst file, however he wants me to configure the users to automatically start saving to a .pst file so that way it is ready to import when we make the change over. How do I do this? I know that Outlook saves to a .ost file normally, not sure how to make it keep a updated .pst file automatically.


Normally, Outlook will use .ost to save database when you configure IMAP or Exchange account. If you want to use .pst file, we can recreate outlook profile and change the data path, for your reference: http://support.microsoft.com/kb/2752583/en-us If you want to keep your data from your old account then you need to have a .PST file, you can only perform a move or copy, or convert the file. See if the following article would help: http://www.ask.com/answers/611260301/is-there-a-way-to-import-my-microsoft-office-outlook-personal-folders-pst-or-offline-folder-ost-files-into-the-new-installation?qsrc=14106


Outlook uses OST files as a local cache of Exchange mailboxes when connected to an Exchange server using MAPI or Outlook Anywhere; it instead uses PST files when connected to POP3/IMAP4 accounts (*). This local cache can also be disabled for Exchange mailboxes, as others have mentioned.

You could force Outlook to download all messages to a PST file by configuring it to connect to Exchange using POP3/SMTP instead of MAPI or Outlook anywhere, but this would make most Exchange features (like GAL, calendar sharing, shared mailboxes, public folders, and so on) unusable; it's a highly not-recommended configuration.

There is, however, a better option for exporting Exchange mailboxes to PST files: you can do that on the Exchange server itself, using a Mailbox Export Request; you can also do that in bulk, exporting several (or all) of your mailboxes.

(*) In Outlook 2013, OST files are used for IMAP4 too, which makes sense, since IMAP4 keeps the messages on the server and accesses them remotely, instead of downloading them like POP3.


I know that Outlook saves to a .ost file normally - Just a point of clarification: OST files are only used when Outlook is connecting to Exchange Server using cached mode, which isn't the default AFAIK, so Outlook doesn't normally save to an OST file, it does so if it's configured in cached mode, so don't take that for granted.

If you want the email to be saved to a PST file rather than an OST file then you'll need to configure Outlook to connect to the Exchange server using POP instead of MAPI or RPC over HTTP(S).

  • FYI: cached mode is actually the default, you need to turn it off explicitly if you wish so. – Massimo Jun 11 '15 at 18:31
  • Oh. Is it dependent on the Outlook version? I don't remember it being the default. – joeqwerty Jun 11 '15 at 20:20
  • At least since 2010, probably 2007 too. – Massimo Jun 11 '15 at 20:42

You can archive to pst and set all new mail to go into that pst but would need to deactivate any move rule the user has setup. In Account settings you can change the folder where new emails are delivered to. There you can specify a new pst or a existing pst and new emails will be delivered to the inbox of that pst. The setting is under File -> Account settings -> Account Settings -> Change folder. You may also need to notify the user that the email will show up in a different folder. Other ways to kind of do the same thing is auto archive setting of 1 day and running everyday or creating a rule to move incoming files instead of changing the account settings. While not perfect it will shorten the time it takes to archive the remaining items when its ready to move.


All the other answers sticks to the idea, that Exchange Server mailboxes needs to be exported into .pst in order to migrate them to Office 365 via PST importing by network upload. The original question states that data is collected to PST files with Outlook on individual computers, which leads to believe that users should migrate their own data with import tool on Outlook. The difference between OST and PST is well discussed here, and Massimo is closest to best practices with Mailbox Export Requests.

However, intermediate PST files aren't necessary in most cases. Office 365 has several tools to migrate mailbox contents directly from on-premises Exchange:

There might be some kind of limitations with the 3rd party Exchange provider mentioned that have led to using PST files in the first place, e.g. if they have many companies on one AD and do not want to allow administrative access to them all for a migration of an individual client.

In migration cases where you can't use administrative account on the source system, it is possible to use Application Impersonation instead and limit it to specific OU so that you don't have more privileges than you actually need. That should be fine with a (sane) 3rd party email provider. Example:

New-ManagementRoleAssignment -Name:Office365Migration -Role:ApplicationImpersonation   `
    -RecipientOrganizationalUnitScope "example.com/Client/Users"   `

(IMAP migration also exist, but it is way worse than using PST files as it needs passwords from all users in the source system and won't transfer anything else than Inbox (doesn’t migrate contacts, calendar items, tasks or emails bigger than 35 MB). Just do not use it if you already have Exchange.)

If there are any doubt that there might be some problem with direct migration (caused by contracts or configurations with the 3rd party email provider), I'd first take PST backups with Mailbox Export Requests before starting the direct migration. Then you'd have a backup if something is missing.

I understand that this migration has already been done in 2015, but this is generally relevant to anyone who wanders to this page while planning similar migration.

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