When I removed my font styling from the <td> the horizontal line disappeared.
After some investigation I fixed the issue by changing my line-height. It was originally 15px so changed to 1 (no px). Also 14.9px worked too. interestingly any larger px value retained the horizontal glitch.
Andrea's fix works a treat. One last code amend I would make is that you are declaring "font-family:" twice in all your styles for the heading "THREE COMPLIMENTARY CARTRIDGE PACKS" which is preventing Outlook 2010/13 and Gmail from rendering the correct font.
Have you tried removing the font attributes from the div container? It appears to be overriding the line-height to 64px when I test using inspector in Chrome.