The problem, which affects people using Outlook Express 4.01 for Macintosh, occurs when users set up filters for incoming mail to go to a folder other than the in-box. If the computer's hard disk is near capacity, the incoming mail may vanish.
Microsoft characterized the problem as "rare," but urged Macintosh users to fix it by downloading an upgrade, Mac Outlook Express 4.02. That upgrade also will include a fix to the "buffer overrun" or "long file name" security bug, which affects Outlook Express and other email programs.
Microsoft will post the update to its Outlook Express Macintosh Edition site this afternoon.