So a Mail Message can be created from a Post, and linked to another Mail Message if it is a response

fixup! erp5_interface_post: add interaction workflow to set message-id as reference of Internet Message Posts

erp5_interface_post: add interaction workflow to set message-id as reference of Internet Message Posts

In case of Emails, several posts will be created if there are more than 1 recipients, so trying to cancel duplicates Internet Message Posts is impossible (or some customers will never receive the emails)