To change the message margins in Outlook 2013 please follow these steps.
Step 1. Create a new email message
Open up outlook and click on New Email
Step 2. Click in the body of the email
Step 3. Click on Paragraph Settings
Click on the “Format Text” tab up the top and then click on the little expander icon under the section “Paragraph”
Step 4. Change the Paragraph Settings
Change the Margins by adjusting the Indentation Settings as in the image below.
Step 5. Set as Default
Click on the “Set As Default” button down the bottom of the dialogue box
Changing the Margin settings in Outlook 2013 for composing emails has limited options but doable. Just be aware that whatever settings you put in here may be either ignored by the email client on the other end or may cause readability issues depending on the size of the screen they are using so use cautiously.