To determine how Mailer signals the arrival of new mail:
Choose Notification from the Options menu.
Under "Signal new mail with," specify whether you want Mailer to beep, the Header window to flash, or Mailer to play an audio file when new mail arrives. You can choose to have all three signals if you desire.
Set the number of beeps and flashes by clicking on the up or down arrow under Beeps and Flashes.
To play an audio file, either type the full path name of the file in the Sound File text field, or click Select to specify that file in a File Selection dialog box.