Using the latest build, if you set a post as email_only via admin API, then send it, it seems to unpredictably rewrite URLs contained in the post to the domain from which they were sent.
For example, a link to https://somedomain.com/a_story from a post at https://mysite.com, where I have Ghost self-hosted, becomes a link to https://mysite.com/a-story, rewriting and breaking the link.
The post’s links are fine on my Ghost site, but the URLs are rewritten somehow when sending, but only if the email_only flag it set to true via the API.