How to resolve ‘Unable to expand the folder’ error in Outlook

Outlook gives you the option to add another mailbox in the Outlook folders list. But there are many situations where while accessing additional PST files, you may face a number of error messages such as Unable to expand the folder and related ones. These messages are the indication of PST file corruption due to oversized PST files or any other reason which further make the data inaccessible and even lead to data loss situations.

To demonstrate the above example, let’s consider a situation where you are making an attempt to add another mailbox folder to the Outlook and trying to access the PST files directly from the Outlook folder list, you are unable to do so. You will receive an error message like:

“Unable to expand the folder. The set of folders could not be opened.”

Cause:

There can be two causes for this error:

1. Either you have not set the ‘Folder visible’ option to true in order to use additional PST files.

2. Another reason can be PST file corruption that can occur due to many reasons like virus attack, unexpected system shutdown and the major one is oversized file size limit.

Resolution:

Corresponding to the reasons for the Unable to expand the folder PST error, you many go for the below resolutions:

In case, you have not set the Folder visible option to true then do it by following these steps:

1. Right click on the additional PST file that you want to share or access and go to Properties

2. There, move to the Permissions tab and check the option of folder visible option checkbox and click on OK button.

But in other case, if the problem is due to oversized PST corruption then you can perform archiving or compression of large volume outlook files. But even if the problem persists, you may use PST Splitter tool that will divide the large PST files in smaller partitions based on date, folders, mail ID etc in an efficient manner.