Exchange 5.5 and Activesync 4.1

We have a new IPAQ connecting to an XP desktop running Activesync 4.1.

The mail server runs Exchange 5.5 and when Activesync connects through to it it won’t accept the credentials to authenticate to it.
I definitely have the correct server name/domain and password.

Is there somekind of change in 4.1 of Activesync that it won’t talk to older Exchange boxes anymore?
Am I going to be forced up to Exchange 2003 to get this IPAQ to sync?!