If your lucky, it's an update of default configuration.
To confirm it, list directories in your /var/mail
You should find a directory for a domain, then a directory for your user like:
user@domain.tld > /var/mail/domain.tld/user/
A mail directory should have at least directories : cur, new, tmp
Each file in thoses directories are a single mail.
Check into your ~/.muttrc (if it exist) to define where your mail are stored.
https://wiki.debian.org/Mutt
like:
set mbox_type=maildir
set mbox="~/mail/inbox/"
set spoolfile="~/mail/inbox/"
set folder="~/mail/"
set record="~/mail/sent/"
set postponed="~/mail/postponed/"
If your mails are into /var/mail/domain.tld/user/ use something like this on your .muttrc file
set mbox_type=maildir
set mbox="/var/mail/domain.tld/user/inbox/"
set spoolfile="/var/mail/domain.tld/user/inbox/"
set folder="/var/mail/domain.tld/user/"
set record="/var/mail/domain.tld/user/sent/"
set postponed="/var/mail/domain.tld/user/postponed/"
You can backup the directory with your mailbox, using borg or other software:
https://github.com/kahun/awesome-sysadmin#backups