Set a Custom Email Header

/* Set a Custom Email Header

 * 

 * Original doc: https://wpmailsmtp.com/docs/setting-a-custom-email-header/

*/

function wpmsmtp_custom_mail_header( $args ) {

	if ( ! is_array( $args[ 'headers' ] ) ) {

		$args[ 'headers' ] = explode( "\n", str_replace( "\r\n", "\n", $args[ 'headers' ] ) );

	}

	$args[ 'headers' ][] = 'X-Custom-Header: header_value';

	return $args;

	}

add_filter ( 'wp_mail', 'wpmsmtp_custom_mail_header');