You'll have to ask someone smarter than I to explain it to you, but it's padding on the td that contains your two tables. Fix it with padding: 0, of course, on that td. See this fiddle and look for
this_td_has_padding
Pregunta
This is the url: http://rosariodesign.com/leaducate.php This is for a email newsletter
The gray border is a table, which contains another 2 tables inside. You will see that there is a 1px space between the gray border and the content, what is that?
Solución
You'll have to ask someone smarter than I to explain it to you, but it's padding on the td that contains your two tables. Fix it with padding: 0, of course, on that td. See this fiddle and look for
this_td_has_padding
Otros consejos
Actually, what the other user suggested as a fix it's not really a fix. As you may know, css properties like padding, height, margin, are mostly ignore by the majority of email clients or rendered badly.
My fix would be to add this cellpadding="0" cellspacing="0" border="0"
on your first table (and to any other table - for the rest of your life).
Also check this out http://www.campaignmonitor.com/css/ it's a comprehensive study on what's working where.
Cheers!