<!-- inspired by the image from https://webdesign.tutsplus.com/tutorials/htmlcss-tutorials/build-an-html-email-template-from-scratch/ -->

<div class="envelope">
  <div class="paper"></div>
</div>

body {
  background: #70bbd9;
  padding: 50px;
}

.envelope {
  position: relative;
  margin: 50px auto 0;
  width: 98px;
  height: 66px;
  background: #c94548;
  border-radius: 0 0 5px 5px;
  box-shadow: 0 0 1px #c94548 inset;
}
.envelope:before,
.envelope:after {
  content: '';
  position: absolute;
  width: 0;
  height: 0;
  border: 0 solid transparent;
  border-width: 33px 49px;
}
.envelope:before {
  border-bottom-color: #c94548;
  top: -100%;
  left: 0px;
}
.envelope:after {
  border-right-color: #ed4c50;
  border-left-color: #fa565a;
  border-bottom-color: #fa565a;
  top: 0;
  border-radius: 0 0 5px 5px;
  transform: rotate(360deg);
  box-shadow: 0 1px 1px rgba(0,0,0,0.25);
}

.paper {
  background: #fff;
  width: 87px;
  height: 66px;
  margin: 0 auto;
  border-radius: 5px;
  position: absolute;
  left: 6px;
  top: -33px;
  box-shadow: 0 1px 1px rgba(0,0,0,0.25);
}
.paper:before,
.paper:after {
  content: '';
  position: absolute;
  left: 12px;
  background: #e3f1fc;
  height: 4px;
  border-radius: 5px;
}
.paper:before {
  top: 14px;
  width: 31px;
}
.paper:after {
  right: 12px;
  top: 28px;
  box-shadow:
    0 8px 0 #e3f1fc,
    0 16px 0 #e3f1fc,
    0 24px 0 #e3f1fc,
    0 40px 0 #e3f1fc;
}