Opening old OST from SBS2003 mailbox in Outlook 365

Solution 1:

OST file is the "cached" file for mail that exists in your Exchange mailbox. It's meant for offline use, but you are expected to sync back with a server. Your "true" messages are with the Exchange server (SBS server). I would say the most authoritative way to get these back would be to:

1. Restore the SBS server from last backup
2. Export your mail items to PST file
3. Import PST to Office 365 (or just connect PST in outlook and manually move mail)

If you can't access them in that fashion, I would continue on with your 3rd party app, if it can extract those to EML, or possibly PST than you can upload those to O365 thru Outlook.

As far as a conflict, you won't have a problem restoring the server. It will probably be easier to export the PST file from the Exchange console. Just google for how to export mailboxes from Exchange 2003 and you will find guidance on how to do that. Then you can just add the PST file to your Outlook and upload to Office 365 without reconfiguring it.

Solution 2:

An OST is linked to an Outlook profile. So if you still have the Outlook profile you could disable the network connection and open Outlook offline, then export to a PST file. The PST file can then be imported into the new mailbox.