In some cases there maybe a need to add a custom header to emails generated from ServiceNow platform. This can be achieved via business rules.

To add custom header, do the following:

1. Create a business rule on the sys_email table ("on before", "insert/update")   
2. The script is below. It is important to put your headers BEFORE ServiceNow headers.
   If you simply append your headers, they will be cut off.
   
   /* add headers */
   
   if(current.headers.indexOf("X-My-Header-Here") < 0){
          current.headers = "X-My-Header-Here: " + headerValue + "\n" + current.headers;
   }