Mdmd's screen is found here:
1. First, have you got your top menu bar UNminimised. - Right click the top white bar and, if there is no "tick" in the box, click "Minimize the ribbon"
2. Click "File" on the left of the Menu bar.
3. Click "Options" and then, Over on the right of the drop down, click "Mail"
Select the "General" tab and you will see MDMD's picture, with the option.
But, to generally check, whenever you open the mail program, there is an item on the top menu, which you should now see, "Send/Receive messages". Click the small arrow and select your option.