Thanks to glin for these instructions.

1. Download our certificate and save it in a file.
2. Go to Start->Run and run C:\windows\system32\certmgr.msc
3. Right Click on the "Trusted Root Certification Authorities" and select All Tasks->Import...
4. Follow the Certificate Import Wizard to import the certificate file created above.
5. At the Certificate Store screen, select "Automatically select the certificate store based on the type of certificate".
6. Check the Fingerprint below.
7. Continue and finish the Import process.
8. Now, try to your email client again.