Recently I had to update the SSL certificate on my Exchange Server. It is a Unified Communication Certificate from Digicert. Digicert is one of the three listed vendors on the Microsoft knowledge base article 929395. It had been working for almost a year, so it shouldn't be too much of a problem.
To request a new certificate (in Exchange Server 2010) open the Exchange Management Console, navigate to the Servers Container, select the appropriate client Access Server and in the Actions Pane select "Create new Exchange Certificate...".
Follow the wizard and when you're finished you have this request file located on your hard disk. So, I went to the Digicert website, logged on, submitted the request file and within 10 minutes I got a new certificate.
I went back to the Exchange Management Console and finished the wizard to install the SSL certificate and assigned the Exchange services to the new certificate. Checked Outlook Web App and Outlook Anywhere, both were working fine.
After an hour I received a text (SMS) message from another user: "Activesync not working anymore?". Yes, the Windows Mobile devices generated an error regarding certificates. Now, OWA and Outlook Anywhere were working fine, so the certificate was ok.
Checked the Digicert Website using Digicert's help. This will check your certificate and the certificate chain. It turned out that the certificate chain was broken and a new intermediate and root certificate were needed on the Client Access Server (this is not an Exchange issue however, but it is caused by some changes at the Certificate Authority).
I downloaded both the new Intermediate and the Root Certificate from the Digicert website and installed them on the Client Access Server (don't forget you have to install these using the certificates snap-in in the MMC console). After installing these certificates everything was running fine again.