Я понял это! Добавил еще несколько мобильных стилей и подправил код вокруг кнопки CTA.
/*Responsive screens*/
@media screen and (max-width: 500px) {
.stack-column, .stack-column-center { display: block !important;width: 100% !important;max-width: 100% !important;direction: ltr !important; padding-left:0px !important; padding-right:0px !important; padding-bottom:0px !important }
.stack-column-center { text-align: center !important; padding-left:0px !important; padding-right:0px !important; }
.center-on-narrow { text-align: center !important;display: block !important;margin-left: auto !important;margin-right: auto !important;float: none !important; }
table.center-on-narrow { display: inline-block !important; }
.email-container p { font-size: 17px !important; }
.middle-article-mobile-pt { padding-top: 60px !important; }
.middle-article-mobile-pb { padding-bottom: 60px !important; }
.text-right-mobile { padding: 0px !important; }
.body-copy, .body-copy-2, .body-h1, .body-h2, .light-blue-button, .orange-button, .mobile-center { text-align:center !important; }
}
<tr>
<td dir="ltr" height="100%" style="padding: 30px 50px; background-color: #ffffff;" valign="top" width="100%" ><!--[if mso]>
<table align="center" role="presentation" border="0" cellspacing="0" cellpadding="0" width="500" style="width: 500;">
<tr>
<td valign="top" width="500" style="width: 500;">
<![endif]-->
<table align="center" border="0" cellpadding="0" cellspacing="0" role="presentation" style="max-width:500px; padding:0; margin:0;" width="100%">
<tbody>
<tr style="padding:0;margin:0;">
<td align="center" style="font-size:0; padding: 0 0 60px 0;border-bottom:1px solid #ebebeb;" valign="top" class=""><!--[if mso]>
<table role="presentation" border="0" cellspacing="0" cellpadding="0" width="500" style="width: 500;">
<tr>
<td valign="top" width="200" style="width: 200px;">
<![endif]-->
<div class="stack-column" style="display:inline-block; margin: 0; max-width: 200px; vertical-align:top; width:100%;">
<table border="0" cellpadding="0" cellspacing="0" role="presentation" width="100%">
<tbody>
<tr>
<td dir="ltr" style="padding: 0 10px 0 10px;" class=""><img alt="alt_text" border="0" class="center-on-narrow" height="" src="http://go.pardot.com/l/190862/2019-04-09/h9rldk/190862/77786/template_testing_200px_placeholder.png" style="width: 100%; max-width: 200px; height: auto; background: #dddddd; font-family: sans-serif; font-size: 15px; line-height: 15px; color: #555555;" width="200"></td>
</tr>
</tbody>
</table>
</div>
<!--[if mso]>
</td>
<td valign="top" width="300" style="width: 300px;">
<![endif]-->
<div class="stack-column" style="display:inline-block; margin: 0; max-width:300px; vertical-align:top;">
<table border="0" cellpadding="0" cellspacing="0" role="presentation" width="100%">
<tbody>
<tr>
<td class="center-on-narrow text-right-mobile" dir="ltr" style="font-family: sans-serif; font-size: 15px; line-height: 20px; color: #555555; padding: 0 0 0 20px; text-align: left;">
<p class="body-h1" style="margin:0;">Headline</p>
<p class="body-h2" style="margin:0;"><a href="##" style="text-decoration:underline;">Author, CPA<br>
Title title title title</a></p>
<p class="body-copy" style="margin: 15px 0;">Maecenas sed ante pellentesque, posuere leo id, eleifend dolor. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos.</p>
<!-- Button : BEGIN -->
<table border="0" cellpadding="0" cellspacing="0" >
<tbody>
<tr>
<td>
<table align="left" border="0" cellpadding="0" cellspacing="0" width="100%">
<tbody>
<tr>
<td bgcolor="#05090c7" pardot-data="" style="border-radius: 2px; background: rgb(255,255,255);" class=""><a class="light-blue-button" href="##"><!--[if mso]> <![endif]-->Read the Article <!--[if mso]> <![endif]--></a></td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
<!-- Button : END --></td>
</tr>
</tbody>
</table>
</div>
<!--[if mso]>
</td>
</tr>
</table>
<![endif]--></td>
</tr>
</tbody>
</table>
<!--[if mso]>
</td>
</tr>
</table>
<![endif]--></td>
</tr>