The email text on the screen of my MacBook is tooo small to look at. Text is Arial 12, so normally fine but it appears too small to be read properly for me. Is there a function like a magnifier?

Press the keyboard shortcut Command and + (the plus key) to enlarge the contents of the preview pane, including the text of the email. Repeat the keyboard shortcut as many times as you like. Each time, the text gets bigger.

Alternatively, you can use zoom.

To activate Zoom on an iMac, MacBook Air, or MacBook Pro:

1. Navigate to System Preferences. This can be accessed through the Apple menu in the top left-corner, or by typing "System Preferences" in Spotlight.
2. Select Accessibility.
3. Select Zoom from the Vision menu on the left.
4. Select Use keyboard shortcuts to zoom, if you'd like to use your keyboard to control the Zoom functions. Select Use scroll gesture with modifier keys too zoom, if you'd like to use your keyboard in combination a mouse or trackpad to zoom in and out.

Mine is set to 18 on 21.5" iMac & 24 on 27" iMac.