Hi,
Do you use Microsoft Outlook as your mail client?
What's special about the specific folder?
Please check if the Purge option is set correctly in your IMAP account. To check this, go to File > Account Settings > Account Settings, double click your IMAP account. Click Advanced tab and make sure the purge option is set correcly:

In addition, please also logon to the webmail interface of your email account and check how the purge option is set in the webmail interface.
Regards,
Steve Fan
TechNet Community Support
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