If bind9 don’t permit to create SOA records anywhere in the DNS tree (and it seems to be coded like it since a long time) and if Yahoo® use it to consider unwelcome emails sent by a domain without SOA records, I suppose we won’t be able to change Yahoo’s mind nor Bind9.
Good. We don’t have to change our entire software stack 
Not good. We also have to tell that the free domains won’t work to send emails. 
And now, I have to reconsider the threshold of self-hosting if I still hope to host my own mail server.
- buy a domain name
- find some not so badly reputed ipv4 and v6
- rely on the good work of Yunohost about the mails and DNS configuration suggestions
- keep telling the rest of the world to stop using those f*** bastards who breaks Internet but whatever, they will continue.