参考:https://guides.rubyonrails.org/action_mailer_basics.html

user_mailer.rb :

 class UserMailer < ApplicationMailer
  2   default from: ' web3@163.com'
  3
  4   def send_validation_email to, code, subject
  5     @code = code
  6     mail to: to, subject: subject
  7   end
  8
  9 end

正文:app/views/user_mailer/send_validation_email.html.erb

<body style="font-size:14px;font-family:arial,verdana,sans-serif;line-height:1.666;padding:0;margin:0;overflow:auto;white-space:normal;word-wrap:break-word;min-height:100px;
  <table width="700" border="0" align="center" cellspacing="0" style="width:700px;">
    <tbody>
      <tr>
        <td>
          <div style="width:700px;margin:0 auto;border-bottom:1px solid #ccc;margin-bottom:30px;">
            <table border="0" cellpadding="0" cellspacing="0" width="700" height="39" style="font:12px Tahoma, Arial, 宋体;">
              <tbody><tr><td width="210"></td></tr></tbody>
            </table>
          </div>
          <div style="width:680px;padding:0 10px;margin:0 auto;">
            <div style="line-height:1.5;font-size:14px;margin-bottom:25px;color:#4d4d4d;">
              <strong style="display:block;margin-bottom:15px;">尊敬的用户:<span style="color:#f60;font-size: 16px;"></span>您好!</strong>
              <strong style="display:block;margin-bottom:15px;">
                【ParaState】
                验证码:<span style="color:#f60;font-size: 24px"><%= @code %></span>,该验证码将于15分钟后失效。
              </strong>
            </div>
          </div>
          <div style="margin-bottom:30px;">
            <p style="color:#747474;">
            </p >
          </div>
          <div style="margin-bottom:30px;">
            <p style="color:#747474;font-size:14px;">
            (工作人员不会向你索取此验证码,请勿泄漏!)
            </p >
          </div>
          <div style="width:700px;margin:0 auto;">
            <div style="padding:10px 10px 0;border-top:1px solid #ccc;color:#747474;margin-bottom:20px;line-height:1.3em;font-size:12px;">
              <p>此为通知邮件,请勿回复<p>
              <p>ParaState团队</p >
            </div>
          </div>
        </td>
      </tr>
    </tbody>
  </table>
</body>