LetsEncrypt Finally Works with Mailserver too.
Figured it out, we need to add these two lines in main.cf located at etc/postfix/main.cf
Or you can also edit postfix config from mailserver settings and add these two lines of code:
smtp_tls_security_level = may
smtpd_tls_security_level = may
Restart postfix and davecot services and it will remove security error message from email reception that server did not encrypt this message.