Very helpful thanks! I found that my Custom Email CSS Styles were causing the problem. Not sure why, they seem pretty kosher to me? Perhaps Discourse doesn’t like my attribute selectors. Discourse Email customisation is lacking 
a, span {
border-radius: 20px !important;
}
img:not([alt="replies"], [alt="likes"]) {
border-radius: 8px !important;
}
div div[style*="margin-bottom:25px"] div[style*="padding-top:5px"] {
background-color: unset !important;
padding-top: 15px !important;
}
img[alt="Company"] {
height: 75px !important;
}