If there is new mail, the program will "beep", and write to the window for each of the new messages;
Mail from <name> -- <subject>
where <name> is either the name of the person sending it, if available (the ARPA 'From:' line) or machine!login where machine is the machine the mail was sent from. If there is no subject, the message "<no subject>" will appear on the screen.
If the message is a priority message (that is, the header contains a line "Priority:"), then the line output will be "PRIORITY mail from ..." rather than just "Mail from".
This program will run forever, and can internally reset itself if mail is deleted from the incoming mailbox while trying to monitor it.
If wnewmail is started up with a filename, it will perform exactly the same, but with the specified file as the one to check rather than the default users mailbox.