I have literally thousands of emails divided into categories using “Topics” in Postbox.
If importing your postbox account didn’t import your existing Topics, then could be eg: those might have to be then manually recreated again in eM Client, as that part might not be compatible.
All I can suggest is to try updating to the latest V10 for PC or Mac via the release history page and then try the import again. Close eM Client before updating / instaling the update.
Before updating make a manual backup of eM Client via “Menu / Backup” incase you need to restore for any reason. You can see when the backup is complete in Show Operations via clicking the drop-down on the right of Refresh.
If that still doesn’t import your Postbox topics, and you have purchased eM Client Personal or Business version, then go to the following VIP support page and login with your registered email address and lodge a support request.
Lastly if you have to recreate Topics (hopefully you don’t) see my post link below for information.