Aligning (floating) tables will often cause a few pixels worth of gap, particularly in Outlook. Try adding this to your page:
<style type="text/css">
.msoFix {
mso-table-lspace:-1pt;
mso-table-rspace:-1pt;
}
</style>
From memory, this doesn't remove it fully - there is still a stubborn 1px line, which is fine, unless you are using background colors that make it obvious. To get rid of it completely you'll have to play around with adding a border also, which is kinda silly, but that is Outlook for you...
However, for responsive, I'd recommend you go with the <td>
's instead, and just toggle width:100%; display:block;
in the media query to control them responsively. Here is a basic example