i.mail,
i.mail::after {
 --width: 18px;
 --height: 14px;
 height: 14px;
 display: block;
 box-sizing: border-box;
 border: 2px solid;
}

i.mail {
 position: relative;
 width: 18px;
 overflow: hidden;
 border-radius: 2px;
}

i.mail::after {
 position: absolute;
 left: 0;
 bottom: 3px;
 width: 14px;
 transform: rotate(-45deg);
 content: "";
 border-radius: 3px;
} 

i.mail+input {
  padding-left: 36px;
}