For each mailbox, at a set interval Biff looks for unread messages in the mailbox and flags the presence of new mail by beeping and changing the icon and label of the Biff button. If you configure Biff to watch just one mailbox, it will display on the Biff button the number of new messages in it, if any, and mouse button 3 fetches a list of the messages. If you use multiple mailboxes, then mouse button 3 produces a menu that indicates how many new messages are in each mailbox, and which allows you to fetch a list of the messages in any of them. You can click on a message in the message list to view the message in full. If you like, Biff can also post a list of new messages in a mailbox when they arrive.
All parameters are adjustable in the preferences manager.
Thanks to G. Dezern.