Xlbiff presents enough information to tell you: Is this new mail worth reading right now? And it stops distracting you once you decide.

Xlbiff waits in the background, monitoring your mailbox file or IMAP server (or running your custom check-mail script). When a new message arrives, it invokes the MH scan(1) command (or your custom mail-scanning script) and pops up a window with the output (typically the From and Subject line of each new message). If more mail arrives, xlbiff scans again and resizes its preview window accordingly.

Clicking the left mouse button anywhere in the window causes it to vanish. It will also vanish if the mailbox becomes empty. Xlbiff stays out of your way when there is no new mail and pops up only when something requests your attention.