Note: these keyboard shortcuts do not work in Outlook 2013/2016. Keyboarders can use the Ctrl and plus (+) or minus (-) keys to change the zoom level in increments or Ctrl and the zero (0) key to go to 100%.
When your incoming email is zoomed, the easiest fix is to hold Ctrl as you roll the mouse wheel (this is the likely cause for many people). Then tick the box to remember your preference. You can open the Zoom while reading dialog by clicking on the zoom % in the status bar or look for the Zoom button on the Message menu in an opened message.
To lock the zoom level reading messages, open the Zoom While Reading dialog and tick the box to Remember my preference. If you are an Office 365 subscriber, you can now set Outlook to remember your zoom level.
