Earlier this week, my neighbor called and asked if I could take a look at a strange Outlook problem she’d started experiencing.

When I sat down at her PC, she showed me exactly what was happening: she’d click Outlook, the loading animation would briefly appear, and then… nothing. Outlook never opened.

Outlook Won’t Launch, but Outlook.exe Is Running

I immediately recognized the issue from past experience with both Outlook and Microsoft Access. A quick look in Windows Task Manager confirmed it Outlook.exe was running in the background, but no GUI was displayed indicating it did not close properly when she last exited/closed down the program.

I forcibly terminated the process, relaunched Outlook, and she was back in business.

Or so it seemed.
 

The Problem Comes Back

Later that same day, I received another phone call. The exact same issue had returned.

At this point, I decided to take the problem more seriously. Nothing in her setup had changed, but I went ahead and compacted her PST files and ran ScanPST.exe multiple times on each one until no errors were found. I also checked that both Windows and Office were fully up-to-date.

I launched Outlook again, closed it normally and once again, Outlook.exe refused to terminate, no longer appeared visually and was gone from the taskbar. As long as the process was stuck running in the background, Outlook could not be reopened.

A Red Flag: OneDrive Syncing PST Files

While reviewing her system more closely, I noticed something alarming. OneDrive was configured to back up all of her standard folders: Documents, Desktop, Pictures, and more.

That meant her Outlook PST files were also being synchronized, since Microsoft stores them in the user’s Documents folder by default.

This immediately raised a red flag.

When I opened the OneDrive client, I found sync errors. The files failing to sync?

Her PST files.

She insisted that OneDrive had always worked fine and couldn’t possibly be the cause but my gut told me otherwise.

What the Experts Say About PST Files and OneDrive

To confirm my suspicion, I turned to Experts Exchange and posted a question describing the situation. The response from other IT professionals was unanimous:

The Fix: Move PST Files Out of OneDrive

That confirmed it.

I moved her PST files to a local folder outside of any OneDrive synchronized directories. Since then, Outlook has been running smoothly with no issues.

Final Takeaway

If your Outlook.exe process won’t terminate or Outlook refuses to launch, check where your PST and OST files are stored.

Move them to a local folder that is not synchronized by OneDrive.

Once again, it’s astonishing how Microsoft creates these problems themselves by placing critical application files in folders that are commonly synced to the cloud effectively setting users up for failure.

The question should also be asked, why did this setup work fine for months and month (if not years) and suddenly break when no changes were made to the user’s PC? Obviously, Microsoft has made changes!

Seriously… where is the Outlook development team on this?