It seems you are using Dovecot a similar thing as an IMAP server.
he mbox file contains all the messages of a single mailbox.
It is specific to Dovecot, a 3rd party thing.
https://doc.dovecot.org/configuration_manual/mail_location/mbox/ -> Read the manual.
Probably, you will use doveadm - Dovecot's administration utility to manage these things..