Set any email param
$additional_parameters at http://www.php.net/manual/en/function.mail.php
- Multiple calls will append existing params.
- To remove an existing param, specify NULL as the value.
This function may only be applicable if you don't have other WireMail modules
installed as email params are only used by PHP's
Object instance it was called from (method supports fluent interface).
API reference based on ProcessWire core version 3.0.127