Because,After client sent "AUTH NTLM" to server, The Exchange server 2010 will respond non-base64 encoded string "443 NTLM supported.", Actually The Exchange server 2003 just respond "443" without body.

Client-------------------------->Server
AUTH NTLM
                                         443 NTLM Supported (Non-base64 encoded string)
NTLMSSP....
                                         443 NTLMSSP........
BOB_MESSAGE
                                         235 Auth OK.