There is a known bug in Outlook 2016 (plus some other older versions of the Outlook Desktop app) where seemingly random horizontal lines (usually white) will appear across the whole email.
<body>has a background colour of #000000 then the lines will be black instead of white.
For a full technical explanation, plus some suggested solutions, take a look at this article by Action Rocket.
Though there are some tricks that can be tried on individual emails (described in the article above), none of them are 100% guaranteed and rarely get rid of the problem.
On top of this, when you factor in dynamic and personalised content, you might find that while the issue doesn’t occur on the French version of your email, for example, it does appear on the German version. Or vice versa. Or the customers interested in handbags get the white line in a different part of the email to the belts and scarves enthusiasts.
In summary, it’s pretty much random, we hate it as much as you do, and if we could prevent it, we really would!