If you're having fun with a pre-2007 version of Outlook, or any non-Microsoft product trying to talk MAPI to an Exchange 2010 server you might be interested in knowing that the defaults have now changed in the new version, and it expects traffic to be encrypted. This caught us out today as we'd only tested Outlook 2007 before rolling one of our boxes over to 2010 over the weekend.

Two options;

Set-RpcClientAccess -identity SERVERNAME -EncryptionRequired $false