The Primary Domain SSL for the hostname, website and mail server has no issue. Outlook trust the SSL for the Primary mail server domain.
I was able to install Let's Encrypt SSL for the secondary domain website and mail server which can be verified when you open the child domain because it show "MAIL.ALLCOVEREDBYAC.COM HAS SSL FROM LET'S ENCRYPT.
Your SSL will expire in 89 days".
However, when you connect the secondary domain to Outlook it show that the SSL was not trusted with the following error:
This CA root certificate is not trusted.
Issued to: www.example.com
Issued by: www.example.com
Valid from: 3/3/2020 to 3/1/2030
Please help me to fix it. Thanks.