Thank you for that added information and the screenshots, I believe I see exactly what is happening here. Some versions of outlook read html code differently than most other email clients. This is rarely a problem, but one of the somewhat well documented errors with their code reading, is that sometimes it will have a 1 pixel line after some lines of code.
What that means is basically sometimes outlook will have a line after random objects, usually after an area with a differently colored background, or if you're in our legacy template after tables. That line is actually just the background color showing through because the email editor is not displaying that section at quite the right size (just 1 pixel too small).
I'm afraid there's no guaranteed way of fixing that in outlook, especially since different versions of outlook seem to move that 1 pixel line to different places. The only real solution, is to make your background color white or grey, so that the 'line' is essentially invisible. Background images are not shown through those 1 pixel lines, so it's also possible that you could set your background color to white, and then use one of the background patterns to keep the email from looking too empty.
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