To read the messages in the the mbox file:
To read the messages stored in another file:
mail -f usability
This starts the mail program, which displays the mail messages you have stored in the file usability.
To read the messages stored in a folder:
mail -f +frombob
This starts the mail program, which displays the mail messages that are stored in the folder frombob.
The user has defined a mail environment variable so that folders are automatically created in a particular directory, whenever they save a mail message.
The + (plus sign) tells the mail program to look for the file frombob in the directory defined by this variable.