There are at least couple of ways you can get your emails to be responsive or adaptive.
The first method is to create separate tables for each "column" and have them wrap when your container is too narrow to fit both. (See example 1)
The second method is to create columns in tables, like you normally would, and set the display to block when you want them to collapse into rows. (See example 2)
Example 1:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" name="viewport" content="text/html; charset=utf-8; initial-scale=1" />
<title>Method 1</title>
<style type="text/css">
@media only screen and (max-width:600px) {
table[class=container], table[class=responsiveTable] {width: 100% !important;}
}
</style>
</head>
<body>
<table class="container" border="0" cellspacing="0" cellpadding="0" width="600" align="center">
<tr>
<td>
<table class="responsiveTable" border="0" cellspacing="0" cellpadding="0" width="50%" align="left">
<tr>
<td bgcolor="#dddddd" align="center">FIRST COLUMN</td>
</tr>
</table>
<table class="responsiveTable" border="0" cellspacing="0" cellpadding="0" width="50%" align="left">
<tr>
<td bgcolor="#aaaaaa" align="center">SECOND COLUMN</td>
</tr>
</table>
</td>
</tr>
</table>
</body>
</html>
Example 2:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" name="viewport" content="text/html; charset=utf-8; initial-scale=1" />
<title>Method 2</title>
<style type="text/css">
@media only screen and (max-width:600px) {
table[class=container], td[class=responsiveCell] {width: 100% !important;}
td[class=responsiveCell] {display: block !important;}
}
</style>
</head>
<body>
<table class="container" border="0" cellspacing="0" cellpadding="0" width="600" align="center">
<tr>
<td>
<table border="0" cellspacing="0" cellpadding="0" width="100%">
<tr>
<td class="responsiveCell" bgcolor="#dddddd" width="50%" align="center">FIRST COLUMN</td>
<td class="responsiveCell" bgcolor="#aaaaaa" width="50%" align="center">SECOND COLUMN</td>
</tr>
</table>
</td>
</tr>
</table>
</body>
</html>