SVG inline

Simply copy/paste this into your HTML:

<svg class="humbleicons hi-mail-multi" xmlns="http://www.w3.org/2000/svg" width="24" height="24" fill="none" viewBox="0 0 24 24"><path stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M3 3.75v7.5c0 .414.298.75.667.75H6M3 3.75c0-.414.298-.75.667-.75h10.666c.368 0 .667.336.667.75m-12 0 5.146 4.825a1.222 1.222 0 0 0 1.708 0L15 3.75m0 0V9m6 3.75v7.5c0 .414-.299.75-.667.75H9.667C9.298 21 9 20.664 9 20.25v-7.5m12 0c0-.414-.299-.75-.667-.75H9.667c-.369 0-.667.336-.667.75m12 0-5.146 4.825a1.222 1.222 0 0 1-1.708 0L9 12.75"/></svg>

SVG sprite

Download the sprite file and address the icon by its slug:

<svg class="humbleicons"><use xlink:href="humbleicons.svg#mail-multi"/></svg>