real disk folder and/or files of local folder mail

I have tons of email via gmail imap, so every few month i move from imap folder to local folders, but even if i created different folders by date, i see one only very large file still growing on the hard disk (mail_data.dat).

I dont want this file so large, because the larger the file, the easier it is to break, and because it is more difficult to transport it (backup, migrate, etc).

On Outlook, i just create a new PST file and folders and subfolders on it, how can I do the same with EmClient?
I want to create other mail_data.dat to divide the huge amount of emails between different archives.


P.S.: while moving from an imap folder to a new created and empty local folder, i lost the original emails on imap server, and the new local folder is empty, i lost months of emails :frowning: