Migration of emails (copy entire folder)

I recently switched to a new computer and migrated my Mail application data (copy the entire data folder). However, it seems that not all emails, particularly those stored in folders, have been fully imported to the new device.

Could you please guide me on how to:

1. Confirm whether all emails and folders have been successfully imported and are stored locally on the new computer?

2. Ensure that any missing emails are downloaded or synchronized with the Mail application from my email account/server?

Additionally, if there are specific steps or settings to check for a complete migration, I would greatly appreciate your advice.

Thank you for your assistance.