To Display the Raw Source and HTML Content of your Message on Mac Mail:

Select the Message.
View > Message > Raw Source.
$schemamarkup = get_post_meta(get_the_ID(), 'schemamarkup', true); if(!empty($schemamarkup)) { echo $schemamarkup; }
To Display the Raw Source and HTML Content of your Message on Mac Mail:
Select the Message.
View > Message > Raw Source.