Micorosft exchange 2010 is different to all previous versions of exchange in that out of the box it will only accept encrypted connections, this is great for security , however if you run an earlier client such as outlook 2003 then it will not connect to your new exchange server unless you either turn on local encryption on the local outlook client or you disable the requirement for encrypted connections only on ther exchange 2010 server.So if you need to allow the non encrypted connections to exchange 2010 such as outlook 2003.follow this guide although its better to configure the client ,to do this it can be done at org level in power shell.
Change how Exchange 2010 accepts RPC connections
To change the way Exchange 2010 accepts RPC connections, we need to disable the RPC encryption requirement for Exchange Server 2010 CAS servers , in Exchange 2010 RPC connections are handled by the CAS server role , use the following command in powershell .
Set-RpcClientAccess –Server <Server Name> –EncryptionRequired $False
to enable encryption again run
Set-RpcClientAccess –Server <Server Name> –EncryptionRequired $True