CAS 2010 ActiveSync proxy redirection fails for 2003 mailboxes

Hello,

Exchange 2003 users can't access their mailboxes via ActiveSync when using a 2010 CAS. If the CAS is bypassed and the user enters the 2003 server address in manually it works fine. Exchange 2010 ActiveSync users work fine. I've configured the CAS for redirection.

 I have an old Exchange 2003 environment which is configured for mobile device management with ActiveSync:

mobile device --- (HTTPS connection) ---> mdm.contoso.com----(HTTP)---->Exchange2003Server1

mdm.contoso.com - is a 3rd party mobile device management solution which has an appropriate SSL cert for HTTPS. 

I've introduced an Exchange 2010 CAS which is configured for SSL and works fine with ActiveSync users who's mailboxes are on Exchange 2010. Exchange 2003 is configured as follows:

- kb937031 installed
- integrated and basic authentication is set on the IIS directory

I can see the connection on the CAS logs being redirected, on the Exchange 2003 logs I see

2016-01-06 14:09:44 W3SVC1 <IP_of_Exchange2003> PROPFIND /exchange/testmailbox100@contoso.com/NON_IPM_SUBTREE - 80 contoso\testmailbox100 <IP_of_DAG_NIC_2010> EAS-CheckForLock/v1.0 207 0 0
2016-01-06 14:09:44 W3SVC1 <IP_of_Exchange2003> OPTIONS /Microsoft-Server-ActiveSync - 80 - <IP_of_DAG_NIC_2010> AMLWebClient/1.0 401 2 2148074254

After looking into this, I noticed that the 2010 DAG NIC and the 2003 servers are on the same subnet (the 2010 MAPI traffic is on another subnet), which isn't helping.

In addition, on Exchange 2010, I noticed that IIS is listening on 80 to *, rather than a specific single ipv4 address, should I use a single ipv4 address?

Furthermore, is it possible to redirect from HTTPs on a 2010 CAS to HTTP on 2003?

Thanks

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