Style your emails for responsiveness using Zurb Ink framework. I have customized it to some extent to wrap things in a container. I am also using premailer rails gem to inline my external styles.

email.css

/**********************************************
* Ink v1.0.5 - Copyright 2013 ZURB Inc        *
**********************************************/

/* Client-specific Styles & Reset */

#outlook a {
  padding:0;
}

body{
  width:100% !important;
  min-width: 100%;
  -webkit-text-size-adjust:100%;
  -ms-text-size-adjust:100%;
  margin:0;
  padding:0;
  color: #333;
}

.ExternalClass {
  width:100%;
}

.ExternalClass,
.ExternalClass p,
.ExternalClass span,
.ExternalClass font,
.ExternalClass td,
.ExternalClass div {
  line-height: 100%;
}

#backgroundTable {
  margin:0;
  padding:0;
  width:100% !important;
  line-height: 100% !important;
}

img {
  outline:none;
  text-decoration:none;
  -ms-interpolation-mode: bicubic;
  /*width: auto;*/
  /*max-width: 100%;*/
  float: left;
  clear: both;
  display: block;
}

center {
  width: 100%;
  min-width: 580px;
}

a img {
  border: none;
}

p {
  margin: 0 0 0 10px;
}

table {
  border-spacing: 0;
  border-collapse: collapse;
}

td {
  word-break: break-word;
  -webkit-hyphens: auto;
  -moz-hyphens: auto;
  hyphens: auto;
  border-collapse: collapse !important;
}

table, tr, td {
  padding: 0;
  vertical-align: top;
  text-align: left;
}

hr {
  color: #d9d9d9;
  background-color: #d9d9d9;
  height: 1px;
  border: none;
}

/* Responsive Grid */

table.body {
  height: 100%;
  width: 100%;
}

table.container {
  width: 580px;
  margin: 0 auto;
  text-align: inherit;
}

table.row {
  padding: 0px;
  width: 100%;
  position: relative;
}

table.container table.row {
  display: block;
}

td.wrapper {
  padding: 10px 20px 0px 0px;
  position: relative;
}

table.columns,
table.column {
  margin: 0 auto;
}

table.columns td,
table.column td {
  padding: 0px 0px 10px;
}

table.columns td.sub-columns,
table.column td.sub-columns,
table.columns td.sub-column,
table.column td.sub-column {
  padding-right: 10px;
}

td.sub-column, td.sub-columns {
  min-width: 0px;
}

table.row td.last,
table.container td.last {
  padding-right: 0px;
}

table.one { width: 30px; }
table.two { width: 80px; }
table.three { width: 130px; }
table.four { width: 180px; }
table.five { width: 230px; }
table.six { width: 280px; }
table.seven { width: 330px; }
table.eight { width: 380px; }
table.nine { width: 430px; }
table.ten { width: 480px; }
table.eleven { width: 530px; }
table.twelve { width: 580px; }

table.one center { min-width: 30px; }
table.two center { min-width: 80px; }
table.three center { min-width: 130px; }
table.four center { min-width: 180px; }
table.five center { min-width: 230px; }
table.six center { min-width: 280px; }
table.seven center { min-width: 330px; }
table.eight center { min-width: 380px; }
table.nine center { min-width: 430px; }
table.ten center { min-width: 480px; }
table.eleven center { min-width: 530px; }
table.twelve center { min-width: 580px; }

table.one .panel center { min-width: 10px; }
table.two .panel center { min-width: 60px; }
table.three .panel center { min-width: 110px; }
table.four .panel center { min-width: 160px; }
table.five .panel center { min-width: 210px; }
table.six .panel center { min-width: 260px; }
table.seven .panel center { min-width: 310px; }
table.eight .panel center { min-width: 360px; }
table.nine .panel center { min-width: 410px; }
table.ten .panel center { min-width: 460px; }
table.eleven .panel center { min-width: 510px; }
table.twelve .panel center { min-width: 560px; }

.body .columns td.one,
.body .column td.one { width: 8.333333%; }
.body .columns td.two,
.body .column td.two { width: 16.666666%; }
.body .columns td.three,
.body .column td.three { width: 25%; }
.body .columns td.four,
.body .column td.four { width: 33.333333%; }
.body .columns td.five,
.body .column td.five { width: 41.666666%; }
.body .columns td.six,
.body .column td.six { width: 50%; }
.body .columns td.seven,
.body .column td.seven { width: 58.333333%; }
.body .columns td.eight,
.body .column td.eight { width: 66.666666%; }
.body .columns td.nine,
.body .column td.nine { width: 75%; }
.body .columns td.ten,
.body .column td.ten { width: 83.333333%; }
.body .columns td.eleven,
.body .column td.eleven { width: 91.666666%; }
.body .columns td.twelve,
.body .column td.twelve { width: 100%; }

td.offset-by-one { padding-left: 50px; }
td.offset-by-two { padding-left: 100px; }
td.offset-by-three { padding-left: 150px; }
td.offset-by-four { padding-left: 200px; }
td.offset-by-five { padding-left: 250px; }
td.offset-by-six { padding-left: 300px; }
td.offset-by-seven { padding-left: 350px; }
td.offset-by-eight { padding-left: 400px; }
td.offset-by-nine { padding-left: 450px; }
td.offset-by-ten { padding-left: 500px; }
td.offset-by-eleven { padding-left: 550px; }

td.expander {
  visibility: hidden;
  width: 0px;
  padding: 0 !important;
}

table.columns .text-pad,
table.column .text-pad {
  padding-left: 10px;
  padding-right: 10px;
}

table.columns .left-text-pad,
table.columns .text-pad-left,
table.column .left-text-pad,
table.column .text-pad-left {
  padding-left: 10px;
}

table.columns .right-text-pad,
table.columns .text-pad-right,
table.column .right-text-pad,
table.column .text-pad-right {
  padding-right: 10px;
}

/* Block Grid */

.block-grid {
  width: 100%;
  max-width: 580px;
}

.block-grid td {
  display: inline-block;
  padding:10px;
}

.two-up td {
  width:270px;
}

.three-up td {
  width:173px;
}

.four-up td {
  width:125px;
}

.five-up td {
  width:96px;
}

.six-up td {
  width:76px;
}

.seven-up td {
  width:62px;
}

.eight-up td {
  width:52px;
}

/* Alignment & Visibility Classes */

table.center, td.center {
  text-align: center;
}

h1.center,
h2.center,
h3.center,
h4.center,
h5.center,
h6.center {
  text-align: center;
}

span.center {
  display: block;
  width: 100%;
  text-align: center;
}

img.center {
  margin: 0 auto;
  float: none;
}

.show-for-small,
.hide-for-desktop {
  display: none;
}

/* Typography */

body, table.body, h1, h2, h3, h4, h5, h6, p, td {
  color: #222222;
  /*font-family: "Helvetica", "Arial", sans-serif; */
  font-weight: normal;
  padding:0;
  margin: 0;
  text-align: left;
  line-height: 1.3;
}

h1, h2, h3, h4, h5, h6 {
  word-break: normal;
}

h1 {font-size: 40px;}
h2 {font-size: 36px;}
h3 {font-size: 32px;}
h4 {font-size: 28px;}
h5 {font-size: 24px;}
h6 {font-size: 20px;}
body, table.body, p, td {font-size: 14px;line-height:19px;}

p.lead, p.lede, p.leed {
  font-size: 18px;
  line-height:21px;
}

p {
  margin-bottom: 10px;
}

small {
  font-size: 10px;
}

a {
  color: #59c1f4;
  text-decoration: none;
}


h1 a,
h2 a,
h3 a,
h4 a,
h5 a,
h6 a {
  color: #59c1f4;
}

h1 a:active,
h2 a:active,
h3 a:active,
h4 a:active,
h5 a:active,
h6 a:active {
  color: #59c1f4 !important;
}

h1 a:visited,
h2 a:visited,
h3 a:visited,
h4 a:visited,
h5 a:visited,
h6 a:visited {
  color: #59c1f4 !important;
}

/* Panels */

.panel {
  background: #f2f2f2;
  border: 1px solid #d9d9d9;
  padding: 10px !important;
}

.sub-grid table {
  width: 100%;
}

.sub-grid td.sub-columns {
  padding-bottom: 0;
}

/* Buttons */

table.button,
table.tiny-button,
table.small-button,
table.medium-button,
table.large-button {
  width: 100%;
  overflow: hidden;
}

table.button td,
table.tiny-button td,
table.small-button td,
table.medium-button td,
table.large-button td {
  display: block;
  width: auto !important;
  text-align: center;
  background: #59c1f4;
  border: 1px solid #59c1f4;
  color: #ffffff;
  padding: 8px 0;
}

table.tiny-button td {
  padding: 5px 0 4px;
}

table.small-button td {
  padding: 8px 0 7px;
}

table.medium-button td {
  padding: 12px 0 10px;
}

table.large-button td {
  padding: 21px 0 18px;
}

table.button td a,
table.tiny-button td a,
table.small-button td a,
table.medium-button td a,
table.large-button td a {
  font-weight: bold;
  text-decoration: none;
  /*font-family: Helvetica, Arial, sans-serif;*/
  color: #ffffff;
  font-size: 16px;
}

table.tiny-button td a {
  font-size: 12px;
  font-weight: normal;
}

table.small-button td a {
  font-size: 16px;
}

table.medium-button td a {
  font-size: 20px;
}

table.large-button td a {
  font-size: 24px;
}

table.button:hover td,
table.button:visited td,
table.button:active td {
  background: #2795b6 !important;
}

table.button:hover td a,
table.button:visited td a,
table.button:active td a {
  color: #fff !important;
}

table.button:hover td,
table.tiny-button:hover td,
table.small-button:hover td,
table.medium-button:hover td,
table.large-button:hover td {
  background: #2795b6 !important;
}

table.button:hover td a,
table.button:active td a,
table.button td a:visited,
table.tiny-button:hover td a,
table.tiny-button:active td a,
table.tiny-button td a:visited,
table.small-button:hover td a,
table.small-button:active td a,
table.small-button td a:visited,
table.medium-button:hover td a,
table.medium-button:active td a,
table.medium-button td a:visited,
table.large-button:hover td a,
table.large-button:active td a,
table.large-button td a:visited {
  color: #ffffff !important;
}

table.secondary td {
  background: #e9e9e9;
  border-color: #d0d0d0;
  color: #555;
}

table.secondary td a {
  color: #555;
}

table.secondary:hover td {
  background: #d0d0d0 !important;
  color: #555;
}

table.secondary:hover td a,
table.secondary td a:visited,
table.secondary:active td a {
  color: #555 !important;
}

table.success td {
  background: #5da423;
  border-color: #457a1a;
}

table.success:hover td {
  background: #457a1a !important;
}

table.alert td {
  background: #c60f13;
  border-color: #970b0e;
}

table.alert:hover td {
  background: #970b0e !important;
}

table.radius td {
  -webkit-border-radius: 3px;
  -moz-border-radius: 3px;
  border-radius: 3px;
}

table.round td {
  -webkit-border-radius: 500px;
  -moz-border-radius: 500px;
  border-radius: 500px;
}

/* Outlook First */

body.outlook p {
  display: inline !important;
}

/*  Media Queries */

@media only screen and (max-width: 600px) {

  table[class="body"] img {
    width: auto !important;
    height: auto !important;
  }

  table[class="body"] center {
    min-width: 0 !important;
  }

  table[class="body"] .container {
    width: 95% !important;
  }

  table[class="body"] .row {
    width: 100% !important;
    display: block !important;
  }

  table[class="body"] .wrapper {
    display: block !important;
    padding-right: 0 !important;
  }

  table[class="body"] .columns,
  table[class="body"] .column {
    table-layout: fixed !important;
    float: none !important;
    width: 100% !important;
    padding-right: 0px !important;
    padding-left: 0px !important;
    display: block !important;
  }

  table[class="body"] .wrapper.first .columns,
  table[class="body"] .wrapper.first .column {
    display: table !important;
  }

  table[class="body"] table.columns td,
  table[class="body"] table.column td {
    width: 100% !important;
  }

  table[class="body"] .columns td.one,
  table[class="body"] .column td.one { width: 8.333333% !important; }
  table[class="body"] .columns td.two,
  table[class="body"] .column td.two { width: 16.666666% !important; }
  table[class="body"] .columns td.three,
  table[class="body"] .column td.three { width: 25% !important; }
  table[class="body"] .columns td.four,
  table[class="body"] .column td.four { width: 33.333333% !important; }
  table[class="body"] .columns td.five,
  table[class="body"] .column td.five { width: 41.666666% !important; }
  table[class="body"] .columns td.six,
  table[class="body"] .column td.six { width: 50% !important; }
  table[class="body"] .columns td.seven,
  table[class="body"] .column td.seven { width: 58.333333% !important; }
  table[class="body"] .columns td.eight,
  table[class="body"] .column td.eight { width: 66.666666% !important; }
  table[class="body"] .columns td.nine,
  table[class="body"] .column td.nine { width: 75% !important; }
  table[class="body"] .columns td.ten,
  table[class="body"] .column td.ten { width: 83.333333% !important; }
  table[class="body"] .columns td.eleven,
  table[class="body"] .column td.eleven { width: 91.666666% !important; }
  table[class="body"] .columns td.twelve,
  table[class="body"] .column td.twelve { width: 100% !important; }

  table[class="body"] td.offset-by-one,
  table[class="body"] td.offset-by-two,
  table[class="body"] td.offset-by-three,
  table[class="body"] td.offset-by-four,
  table[class="body"] td.offset-by-five,
  table[class="body"] td.offset-by-six,
  table[class="body"] td.offset-by-seven,
  table[class="body"] td.offset-by-eight,
  table[class="body"] td.offset-by-nine,
  table[class="body"] td.offset-by-ten,
  table[class="body"] td.offset-by-eleven {
    padding-left: 0 !important;
  }

  table[class="body"] table.columns td.expander {
    width: 1px !important;
  }

  table[class="body"] .right-text-pad,
  table[class="body"] .text-pad-right {
    padding-left: 10px !important;
  }

  table[class="body"] .left-text-pad,
  table[class="body"] .text-pad-left {
    padding-right: 10px !important;
  }

  table[class="body"] .hide-for-small,
  table[class="body"] .show-for-desktop {
    display: none !important;
  }

  table[class="body"] .show-for-small,
  table[class="body"] .hide-for-desktop {
    display: inherit !important;
  }
}

table.facebook td {
  background: #3b5998;
  border-color: #2d4473;
}

table.facebook:hover td {
  background: #2d4473 !important;
}

table.twitter td {
  background: #00acee;
  border-color: #0087bb;
}

table.twitter:hover td {
  background: #0087bb !important;
}

table.google-plus td {
  background-color: #DB4A39;
  border-color: #CC0000;
}

table.google-plus:hover td {
  background: #CC0000 !important;
}

.template-label {
  color: #ffffff;
  font-weight: bold;
  font-size: 11px;
}

.callout .wrapper {
  padding-bottom: 20px;
}

.callout .panel {
  background: #ECF8FF;
  border-color: #b9e5ff;
}

.header {
  padding: 0;
  background-color: #333333;
}

.footer .wrapper {
  background: #333333;
}

.footer h5 {
  padding-bottom: 10px;
}

table.columns .text-pad {
  padding-left: 10px;
  padding-right: 10px;
}

table.columns .left-text-pad {
  padding-left: 10px;
}

table.columns .right-text-pad {
  padding-right: 10px;
}

.circular img{
  width: 120px;
  height: 120px;
  border-radius: 150px;
  -webkit-border-radius: 150px;
  -moz-border-radius: 150px;
}

img.grid-img{
  height: 80px;
  width: 80px;
  padding: 0px 1px 2px 1px;
}

img.pinvite{
  height: 120px;
  width: 240px;
  /*padding: 1px 1px 1px 1px;*/
}

.footer{
  background: #5e5e5e;
}
.footnav a{
  color: white;
  text-decoration: none;
  font-size: 12px;
}
a#cr{
  cursor: default;
}
div#content-wrapper{
  padding: 30px;
  background-color: #f5f5f5;
}

@media only screen and (max-width: 600px) {

  table[class="body"] .right-text-pad {
    padding-left: 10px !important;
  }

  table[class="body"] .left-text-pad {
    padding-right: 10px !important;
  }
}

mail_layout.html.erb

<!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" content="text/html; charset=utf-8" />
  <meta name="viewport" content="width=device-width"/>
  <link rel="stylesheet" type="text/css" href="email.css">
</head>
<% root_url =  "http://www.yourdomain.co" %>
<body>
  <table class="body">
    <tr>
      <td class="center" align="center" valign="top">
        <center>



          <table class="container">

            <table class="row header">
              <tr>
                <td class="center" align="center">
                  <center>

                    <table class="container">
                      <tr>
                        <td class="wrapper last">

                          <table class="twelve columns">
                            <tr>
                              <td>
                                <a href="<%= root_url %>">
                                  <img src="http://websitesmadeeasy.tv/wp-content/uploads/2015/02/sample-logo-black12.png" width="150px" style="margin-left:15px;">
                                </a>
                              </td>
                              <td class="expander"></td>
                            </tr>
                          </table>

                        </td>
                      </tr>
                    </table>

                  </center>
                </td>
              </tr>
            </table>

            <tr>
              <td>

                <!-- mail content  -->
                <div id="content-wrapper">
                  <%= yield %>
                </div>
                <!-- mail content -->

                <!-- footer starts here -->
                <table class="row" style="padding:0; background-color:#5e5e5e;">
                  <tr>
                    <td class="wrapper footer">

                      <table class="ten columns">
                        <tr>
                          <td style="padding-left:10px">

                            <table class="footnav">
                              <tr><td>
                                <a href="<%= root_url %>about">ABOUT</a> &nbsp;
                                <a href="<%= root_url %>faq">FAQS</a> &nbsp;
                                <a href="<%= root_url %>terms">TERMS</a> &nbsp;
                                <a href="<%= root_url %>privacy">PRIVACY</a>&nbsp;
                                <a id="cr" href="#">&#169; YourCo 2015</a>
                              </td></tr>
                            </table>

                          </td>
                          <td class="expander"></td>
                        </tr>
                      </table>

                    </td>

                    <td class="wrapper last" style="background:#5e5e5e;">

                      <table class="two columns">
                        <tr>
                          <td style="padding-left:10px">

                            <table class="footnav">
                              <tr><td>

                                <a href="https://www.facebook.com/sweatpass" >
                                  <img src="https://sweatpass-staging.herokuapp.com/images/facebook.png">
                                </a>

                                <a href="https://twitter.com/getsweatpass" style="float:left; margin-left:5px;">
                                  <img src="https://sweatpass-staging.herokuapp.com/images/twitter.png">
                                </a>
                              </td></tr>

                            </table>

                          </td>
                          <td class="expander"></td>
                        </tr>
                      </table>

                    </td>
                  </tr>
                </table>
                <!-- footer ends here -->

              </td>
            </tr>
          </table>

        </center>
      </td>
    </tr>
  </table>
</body>
</html>

signedup.html.erb

<table class="row">
  <tr>
    <td class="wrapper last">
      <table class="twelve columns">
        <tr>
          <td>
            Dear <%= @user.first_name %>,
            <br><br>
            <p>We're thrilled that you're joining us.</p>

            <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non
proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>

            <p>
              Until then,.
              <br>Jason Bourne
              <br>Founder, YourCo
            </p>

            </td>
            <td class="expander"></td>
          </tr>
        </table>
      </td>
    </tr>
  </table>