If you have a long list of emails and you’re having difficulty distinguishing among the individual emails in your list, you can select a grid line style to display between each of the emails. This can improve the visibility of each email in your list.
To change this setting, first make sure the Mail module is active. If not, click the Mail link at the bottom of the Outlook window. Then, click the View tab.
Click View Settings in the Current View section of the View tab.
On the Advanced View Settings dialog box, click Other Settings.
Select an option from the Grid line style drop-down list in the Grid Lines and Group Headings section of the Other Settings dialog box.
NOTE: You can also choose not to separate emails by day by selecting the Show items in Groups check box so there is NO check mark in the box.
Click OK.
Click OK on the Advanced View Settings dialog box to close it.
Your chosen grid line style should display as separators between each email in your list.
Notice that when you select an email, a dotted line displays around the email. That can be another way to distinguish an email from others, as well. If you have the Reading Pane turned off (in the Layout section of the View tab), it does no harm to select an email to call it out in the list.
Source: http://www.howtogeek.com/173201/how-to-create-a-clearer-separation-between-each-email-in-your-message-list-in-outlook-2013/
