When using Outlook 2016 (for Windows), you may experience a bug in Outlook that causes a thin line to appear in your template when inserted. In every case that we have discovered online, and from other HTML email experts, these lines have the color of the background.
We are continuing to investigate with other experts in the HTML email field to see if there is a solution to this issue that ensures random lines don't appear when using different backgrounds and content colors.
The current workaround is to ensure your email body background and main content background have the same color you won't see the issue.
Please contact your Account Manager for more information if you are experiencing this issue.