Plesk mailbox:
On Linux, Plesk mailbox is stored in the directory specified in value to parameter PLESK_MAIL NAMES_D in configuration between the file like/etc/psa/psa.conf.
By default, the path is /var/mail/mail names(for both Qmail and Postfix) can be changed the location of mailboxes.
PLESK:
Plesk is a control panel available on Linux-based hosting Accounts and also servers. It’s easily manage many aspects of your account, including the files, applications, and servers, it’s also hosted on your account or server. The operating systems supporting Plesk include Fedora, Red Hat, Debian, FreeBSD, SuSE, CentOS, Ubuntu, and Windows.
Mail Box:
It’s a storage place, a computer file in which email messages received by a particular user are stored.
To limit the size of the Plesk mailbox on Linux:
Plesk bin mail -u JDoe@example.com-m box_quota 50M