To get rid of this notification, export the Exchange certificate and covert it to pem. (key and cert in one file called exchange.pem)
mkdir /var/spool/postfix/certs && chown postfix /var/spool/postfix/certs
In /etc/postfix/main.cf add smtp_tls_CAfile = /var/spool/postfix/cert/exchange.pem & smtp_use_tls = yes
Copy exchange.pem in /var/spool/postfix/certs
c_rehash /var/spool/postfix/certs (you need the openssl-perl package)
chown postfix /var/spool/postfix/certs/exchange.pem
OS Centos 6.2_x64
Note: Probably I rushed to conclusion. The warning still shows in logs. I’ll leave it as it is for now.