Deleted folder doesn't release space

During my attempts of copying folders from an imap account to a local folder I managed to partially copy one folder that was several Gb in size, to estimate total time needed. After about an hour I cancelled the copy operation and in that local folder I saw about 5.000 messages. So far so good.
But then I deleted the local folder to run the actual copy in a later time (to not have duplicate items).
I notice that apparently the space was not released (verifying space under C:).
Is there any scheduled process or manual process to be sure to purge this kind of situation?
Or simply when I run the process it will reutilize that “virtual” free space allocated by emclient?
Thanks in advance,
Gianluca