It was a fairly lengthy setup and debug process...
What's lacking in the Post by Mail is to NOT CHANGE the formatting of the email. or, alternatively provide an option to transfer as plain text.
In our situation case, we need to automate posting of videos via youtube. The Youtube plugin requires plain text (not hyperlink). So, we send plain text to the generated email address - but, the Jetpack system (maybe underlying wordpress email system) is adding hyperlinks.
The post by mail feature works pretty well in many respects, but is missing most of the features provided by other tools (though the others require email fetching).
Maybe they'll fix this and we'll come back.