I’m having the same issue, and I’ve done everything from rebuild the entire email to removing elements and styles one pain staking piece at a time. I’ve been able to make the line go away, as randomly as it appears, but then a day later the exact same code renders that stupid line again. Wish I had a solid fix to share! I have noticed that if I go into my Outlook 2016 inbox (yes, I am a lucky MSO user…) and open the email several times, sometimes the line appears and sometimes it does not. Litmus has never rendered the line in a preview, but EOA does render the same issue in tests as I am seeing live. Hanging around till I find a solution.
Mine appears consistently (every time). I wish I knew why too, perhaps I’ll start to delete elements, that’s a good idea. Do either of you want to share your designs so I can see if there are any similarities? In my design the issue occurs immediately after an Email on Acid bulletproof button. https://www.emailonacid.com/blog/article/email-development/simple-code-based-bulletproof-button
Those are also the only tables in my work that has align=“left” so I was wondering if that could somehow cause it. It is basically the background colour showing through. And that is unusual, however, thinking about it, perhaps something else is being set by the background colour. Now, I know you may think that’s not possible, but what about if somehow it is setting :root or something, as per this article https://blog.gospodarets.com/css_properties_in_depth