Creating a responsive column using e-Newsletter

I've successfully made a custom e-Newsletter template by copying a template to the upload folder
I've created a code like this:
<div style="width: 100%">
<div class="testcolumn">abc</div>
<div class="testcolumn">abc</div>
<div style="clear: both"></div>
Then in the .css style I placed this:
.testcolumn { width: 50%; float: left; }
@media only screen and (max-width:450px)
.testcolumn { width: 100%; float: left; }

But when I do testing when I shrink the window it doesn't go to full width
When I look into the code:
<table align="left" border="0" cellpadding="0" cellspacing="0" class="fullmintywrapper columnWrapper">
This line always added a style="width: 300px" something like that