Hi @Cedric ,
Welcome to our forum!
Based on my research, i didn't found any GPO to set the cache email for IMAP account, but only for Exchange account.
The goal is to reduce the space used by OST files.
And i notice that you want to reduce the size of data file, we can try the following:
Archive in Outlook for Windows
Hope to help you!
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.