Hi @Locker,
1.
Yesterday Mail-in-a-Box don’t approve of installing Let’s Encrypt b/c AAAA is different. Today is Ok, I don’t know why … Lucky for me 
The error is still there: This domain should resolve to your box's IP address (AAAA 2404:f080:1101:0316:a163:0044:0207:050f) if you would like the box to serve webmail or a website on this domain. The domain currently resolves to 2404:f080:1101:316:a163:44:207:50f in public DNS. It may take several hours for public DNS to update after a change. This problem may result from other issues listed here.
I have solutions for it, add more email like demo@mail.my-domain.com and Mail-in-a-Box auto created a new domain in the system and I just installed Let’s Encrypt 
Thank for your help!