I'm currently on Exchange 2003 and have created a test lab to test the migration from Exchange 2003 to 2010. Just for grins I tried exporting the certificate (.pfx with private key) from my current Exchange 2003 server and then imported it into my 2010 server. The status shows as "The certificate is invalid for Exchange Server usage", but if I look at the certificate the chain is "OK". I tried re-importing the root certs from the COMODO website. Also, I can use the EMS command "Enable-ExchangeCertificate certificate -services IIS" to assign it to the IIS service, browse the OWA webpage on the local machine (as I added the URL "mail.domain.com" of the cert to the host file on that system to resolve locally) and it works! I've also set the URLs on the Server Configuration/Client Access services (OWA, ECP..) to the cert domain (mail.domain.com), just in case it was complaining about that, no joy. Thanks all.