var appweburl = "https://example.com/site";
var urlTemplate = appweburl + "/_api/SP.Utilities.Utility.SendEmail";

$.ajax({
    contentType: 'application/json',
    url: urlTemplate,
    type: "POST",
    data: JSON.stringify({
        'properties': {
            '__metadata': { 'type': 'SP.Utilities.EmailProperties' },
            'From': from,
            'To': { 'results': [to] },
            'Body': message,
            'Subject': subject
        }
    }
  ),
    headers: {
        "Accept": "application/json;odata=verbose",
        "content-type": "application/json;odata=verbose",
        "X-RequestDigest": $("#__REQUESTDIGEST").val()
    },
    success: function (data) {
       console.log("An email was sent.");
    },
    error: function (args) {
       console.log("We had a problem and an email was not sent.");
    }
});

'additionalHeaders': {
     'content-type': 'text/html'
}

The property 'additionalHeaders' does not exist on type 'SP.Utilities.EmailProperties'. Make sure to only use property names that are defined by the type.