I can tell when users upgrade to a new wide screen monitor because they ask how to set the margins in email. Composing an email on a wide screen with windows maximized is not the best experience because lines of text are very long. It's more comfortable to read and compose email when the text is only about 6 inches wide. Rather than resizing the window, users decide that moving the margin is the better idea and it's very easy to do but adjusting the margins using the ruler.
Then users discover unintended consequences:
I used the ruler to adjust the margins but when my boss printed out the email, the text printed in a narrow column and used 13 pages.
The problem is not limited to printed email. Recipients who open messages in smaller windows or who use smaller monitors may also see a (badly) formatted message in a narrow column. Fortunately, the margin setting is ignored by the reading pane.
When you adjust the right margin, you aren't making the page a specific width, you are indenting the right edge by a specific amount. If the page is 12 inches wide and you set the right margin to 6 inches, then you are left with 6 inches of space to type in. However, when the overall page width is 8 inches wide, you're left with just 2 inches when the margin is set to 6 inches, resulting in a narrow column of text in printouts or when the message is opened in a smaller window.
Outlook doesn't have an option to set the width of a message - it uses the full width of the message window and wraps text in the window as needed. Trying to force line lengths using margins or the Enter key to break lines only results in readability problems for the recipient. It's better to adjust the width of the compose window if you need to make the lines shorter and easier to read while composing messages.
To check or adjust the margins in Outlook 2010 and up, you need to look at the Paragraph settings and Page Setup. Both the Left and Right Indentation should be set to 0 (zero).
On the Format tab, click the Expander icon to open the Paragraph dialog. (If the ruler is visible, you can point to the indentation triangles and double click to open the dialog instead.)
The Left and Right Indentations should be set to zero. If you want to use indentation, keep it very small. Do not use indentation or margins to make line wrap on wide screen monitors while you are composing them; this will affect the margins and format on the message the recipient receives. Adjust the width of the compose message form instead.
If you change the values, click the Set as Default button at the bottom of the dialog, then choose All documents based on NormalEmail.dotm template and click OK to commit the changes.
The default Margin setting is 72 points or 1 inch (or the equivalent) for top, bottom, left and right margins. To check the settings, you need to open the Page Setup dialog. You can do this with a double click anywhere on the ruler, or in the gray area on the right edge of the ruler (near the red arrow on the right in the screenshots containing the ruler) After resetting the value to 1 inch (or the equivalent), click Set as Default.
In this screenshot, the right margin is NOT set on the default.