Overview
Having a very long tree of subfolders can lead to problems in a Windows environment. You will not be able to expand the custom child folder if the path to it exceeds 260 characters, i.e.
"C:\Program Files (x86)\Kerio\MailServer\store/mail/domain.com/user/INBOX/All PERSONNEL/Employee - Lay off-Terminate-Resign/History - Lay off Term Resign/2017/2017 03-15 Sherlock, Holmes/2017 03-13 John, Watson/2015 01-05 New Hire John, Doe/Spring-All-company-meeting"
This article provides instructions on how to resolve the above issue.
Root Cause
Windows Maximum Path length limitation is causing this issue.
Solution
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Shorten the long path of that specific folder by changing the names of the subfolders manually from the OS level.
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Re-index the affected user mailbox from the Webadmin.
Additional Information
Windows Maximum Path Length Limitation
Kerio Connect installed in Windows