Tools -> Send/Receive Settings -> Define Send/Receive Groups
Disable the scheduled mail polling. It says to poll every N minutes but
what it really does is poll every N*M minutes where N is the number of
minutes of delay and M in the poll count. Unfortunately, zero (0) is
allowed for M so you get N*0, or zero, which means there is a poll
immediately after Outlook loads enough of itself to perform a zeroeth
poll. By turning off scheduling you eliminate this immediate poll but
then you also eliminate subsequent scheduled polls and will have to do
them manually.