A somewhat frequent issue seen by users of Outook 2016 for Windows, is a 1px horizontal white line spanning across their email template. The line may also take on a different colour depending on the template/email background colour.
Despite the issue being widely reported, unfortunately a simple fix is not yet known. A quick Google search can produce many articles and forums with suggested fixes and conversation regarding the issue, but Microsoft's official statement on the issue is currently "this is actually a known issue, and still under investigation."
Note: This problem doesn't happen in Outlook 2016 for Mac because that client doesn't use MS Word for rendering, whereas Outlook 2016 for Windows uses MS Word as its rendering engine and that’s where the problems occur.