Open Outlook 2013/2016 and go to the File tab.
Now, select Open & Export and click Import/Export.
The Import and Export Wizard will appear on the screen. Select Export to a file and click Next.
Select Outlook Data File (.pst) and click Next.
Now, select the IMAP folder you want to backup and click Next.
Select the location where you want to save this file and click Finish.
This way you can easily backup IMAP mailboxes manually