I've used the following to get it fixed and tested, I'm using Plesk 12.5.30mu48
Postfix
To create Postfix-cacerts.pem in my case:
# cat postfix-cert.pem COMODORSADomainValidationSecureServerCA.crt COMODORSAAddTrustCA.crt > postfix-cacerts.pem
(To be precise, my own cert first, then...