It has been requested many times in the past and now it is finally here! The first version of Postie support for Yoast SEO has been released.
The plugin currently supports setting the Focus Keyphrase and Meta Description via shortcodes you place in the email.
For example:
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
[pseo_focuskw keywords="my keywords"]
[pseo_metadesc description="my description"]
Check out the details at Yoast SEO AddOn