I was trying to connect to my Exchange 2010 RC Server with Outlook 2003, but couldn’t connect successfully. I tried with Outlook 2007 & 2010 and was able to connect successfully. That made me search whether Outlook 2003 is a supported client and it is. The error message is “Unable to open your default e-mail folders”
After playing around for a while, I found the solution. Before I jump in to the fix, let me explain a few changes in Exchange 2010 in terms of MAPI connectivity.
All MAPI clients connecting to Exchange 2010 server connects to the mailbox through the CAS Server.
A new service named Exchange RPC Client Access is introduced in 2010 CAS which handles all MAPI connections.
All MAPI clients connect to the mailbox server directly in Exchange 2007.
We can see that this service needs rpc encryption and it is set to True by default.
Same is the case with Outlook 2007 & 2010 profiles! Encryption between Outlook and Exchange is enabled by default, which explains why these clients can connect to Exchange 2010 without any issues.
Outlook 2003 profiles don’t enable encryption by default.
Once I checked the box, I could connect to my 2010 mailbox.
You can also disable the requirement of 2010 CAS servers to have encryption enabled by running Set-RpcClientAccess –server servername –EncryptionRequired $false. This is not recommended though!
If you have too many Outlook 2003 clients trying to connect to a 2010 server, you can enable encryption using Group Policy.