A recent major issue for a customer of mine revealed a new sting in the tail for Outlook users...a new reason to "get out" altogether before it is too late.
As we are all...mostly, no longer using POP to connect to our email servers, you'll find that Outlook doesn't create you one big PST file. It creates you one big OST file. Further detail from Microsoft themselves is here.
NOTE: Nearly everything you do in Outlook is stored in these files. Despite OST files claiming NOT to hold everything AND synchronize with your server, they are still just as enormous as PST files. That's why they slowly bloat and that's why you lose everything when they burst).
I am pretty sure that, in times of yore, that Outlook's PST and OST files were structured the same, or at least in some ways compatible.
I am positive that, in the past, I have imported OST files into Outlook the same way I did (and you still can) import PST files.
Now that is no longer the case.
If you OST file gets orphaned / disconnected from Outlook and you can't reconnect to the server/host email account then you are in a world of pain.
If you experience this pain and start trying to find solutions you'll quickly be taken to very shady looking web sites offering to sell you tools that get your data from the OST and into the PST file format (where you can reconnect it to Outlook or something else).
If this is where I get to for a client, on a client's machine, this is where I stop.
Being forced to consider these tools is something that I distrust deeply.
It is not something that I feel comfortable doing on a machine that is anything but sacrificial and especially not on a customer's machine.
If the tool ends up being as malicious as the hosting web site looks, then you don't want that anywhere near your important machines or data.
These tools always want money from you before you get anything useful.
Sometimes they'll give you a trial by way of limited output or showing you your data without any real access to it.
Then, when you pay, often "activation" is not particularly quick if it happens at all.
There's a good chance you'll do your many and will have compromised your payment method in the process.
So, yeah, I hate getting to this point and it is all thanks to Microsoft half-arsing it.
I cannot fathom why they/Microsoft, the creators of both these file formats, cannot provide a tool to do the job, or better still, have Outlook handle the fricken mess it has created.
But they don't and it doesn't.
So my best recommendation is get the hell out of Outlook now and/or backup your Outlook data properly and thoroughly in some "non Outlook" or "outside of Outlook" way.
Before it is too late. Before "it" happens to you....because it will happen eventually.
I've written about this sort of thing before but now I implore you.
A couple of my previous articles about Outlook pain still live on in this blog.
I've also spoken about backing up your emails better (or even at all as is most often the case). Again this is generic advice but particularly pertinent to Outlook because it fails so completely so often.
Export your contacts to a spreadsheet, do your calendar appointments some other way and manage your tasks with some other tool.
NOTE: Most often I find people are only using Outlook for email. Thousands and thousands of emails. Three quarters of Outlook's features are not used by most people. Are you using the calendar, tasks, contacts (properly/fully) etc.? I bet not. If that's you, consider moving to a FREE email only program like Thunderbird - data comes and goes (imports and exports) from these much better than it does Outlook.
One of my previous articles, that I emailed out in 2016, explained the main reason why Outlook is so horrible - everything lives in one giant PST file.
Since then the strategy of synchronizing email across multiple devices and machines has become commonplace. That's great.
But sadly synchronizing can also mean you synchronize problems. So then you end up with multiple enormous OST files on various machines that also contain problems.
And in the same year I pointed out the necessity to move from Outlook to something like Gmail. [The conspiracy folks love that one ;-)]
If you ignore my advice, or simply don't see it until it is too late, and you get desperate about converting your OST to a PST, make sure you make an effort to find reputable conversion tools and companies and not just what pops up first when you Google it.
Look for reviews from legitimate magazines, blogs, web sites etc.
Try to reconcile what they are saying and see if there's a common candidate (tool) popping out as a viable and legitimate option.
Even then, only try the tool on a test or sacrificial computer if at all possible.
AND, use it in a trial mode first. Don't pay up front.
If your OST file is so badly corrupted that nothing can be retrieved...and you've already paid for the tool...then you'll find that's your money done in most cases.
For my own reference and curiosity I found these web sites reviewing some of these tools:
Good luck, you'll need it.