Press enter after filling the last address field and they keep appearing. If you want to see more fields before they scroll up and out of sight move the mouse to the line below the subject field and when the cursor changes to a double headed arrow left click, hold and drag down. If you have not discovered the Contact Sidebar in the Write window turn that on by pressing F9. Now go down the list and select all the contacts for the email. Select more than one by holding the control key while clicking them. When you have them all selected use one of the Add To buttons at the bottom to address the email.