I get the error "There is no root certificate for this server" when trying to check my e-mail with Mail.app

If you’re using Apple’s Mac OS X and Mail.app and try to check your e-mail via IMAP, you may intermittently get the error:

and:

which can be seen in this screenshot:

Our certificates are valid, signed SSL certificates, so this appears to be an obscure, hard-to-reproduce bug with OS X not being able to authenticate their validity. This can be fixed by installing the necessary SSL root certificates into your OS X installation. Download both these files to your desktop:

• ValiCert Root Certificate
• Intermediate Certificate

Once downloaded, you can install them into MacOS X by dragging the files to the Keychain application (located in /Library/Applications/Utilities).