This is the type of message you might see after you quit the mail program:
&q Saved 1 message in /user9/eucs/erpl08/mbox Held 6 messages in /usr/spool/mail/erpl08
The user erpl08 had seven messages in their mailbox. Before quiting they read one of these messages, which is saved to the file mbox in their home directory.
The other messages are held in their system mailbox.