.newsletter_wysiwyg {
    border: 1px solid #ccc;
    flex: 1;
    -ms-flex-preferred-size: auto;
    margin:0 0 20px 0;
}

.newsletter_wysiwyg iframe {
    background-color: #fff;
    border: 0;
    box-sizing: border-box;
    flex: 1;
    -ms-flex-preferred-size: auto;
    min-height: 400px;
    width: 100%;
    overflow:hidden;
}