Tested on CentOS7.

We have a Inbox which archives all the incoming eMails for testing reason.
To get this amount of data handled, I rotate it with Logrotate.

vi /etc/logrotate.d/mailbox-rotate

/var/spool/mail/archive {
    weekly
    size 10M
    su backup mail
    rotate 1
    compress
    missingok
    nocreate
    postrotate
        /usr/bin/systemctl reload postfix > /dev/null 2>&1 || true
    endscript
}