Outlook 2016 for Windows often adds random lines at the bottom of some table cells. These lines are coloured like the body background colour, and may only be visible if your main content background color is different from your body background colour.
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.
Can it be fixed?
As emerged from Litmus discussion, Microsoft on a Technet discussion on the same strange behaviour, “this is actually a known issue, and still under investigation”.
Note: This is a issue related to the Outlook 2016 for Windows email client, and not your Poppulo Harmony template. There isn't a direct fix but we will keep an eye on any future bug fixes or developments from Microsoft and we will update again once new information is available.