Backing up Mail Users Only?

Hello. I am looking to make a backup of mail accounts only for a username to keep my client happy.

I know this is done in “v-backup-user username” but this compresses and backs up all email accounts for username.

I guess I’m looking to understand where the mail is stored for a domain account (i.e. all users @ and if and how I should copy them safely?

Is this handled by dovecot or exim4?

Thank you for any pointers or direction you are able to provide!

Kind regards,