Came across this one while building a fairly complex and colorful HTML email yesterday. I rarely use tabular layouts (almost exclusively in eBlasts) and while testing it, I noticed it wasn’t Outlook that was giving me trouble, but rather iOS’s mail. Little hairline borders were showing up. Pinching and zooming in and out made them appear and dissapear.
Turns out this is a known issue — one which Apple has yet to address, and it looks like it’s existed dating back to iOS 5, and maybe earlier.