Post by Email?

If you want to post by email and apply access restrictions based on Groups, you would need to customize it. One solution I can imagine is if you allow some kind of capability indicator in the email content and a handler will read it and apply the access restriction. You can have a look at how Groups applies its access restrictions when a post is saved – https://api.itthinx.com/groups/source-class-Groups_Access_Meta_Boxes.html#_save_post