There is probably a program running that is checking your mail and notifiying you when you receive a new e-mail. Is MSN Messenger running? AIM running? Maybe Outlook just isn't closing itself when you close it.
Does it to do this when you first boot your computer? If so, an application is either doing it or you can try configuring your start up items.
Go to start->run->msconfig (i think itll work in 98 haven't used 98 in a long time).. and select "Startup" and if you see Outlook anywhere UNCHECK it.
Restart and send yourself an email.
Also, yes for 1 minute to check for new mail that is for WHEN the program is started