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Outlook 2003 Cached Exchange low bandwidth threshold


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We've lately started seeing something out of normal behavior from our Outlook 2003 installations. Or then it might be that we've never really understood the application to begin with :sneaky:

We use the combination of Outlook 2003, ISA 2006 and Exchange 2007. Clients connect using HTTPS outside of office network and then RPC while in the office. ISA is expecting HTTP basic authentication from the client protected using SSL.

Outlook is configured to use HTTPS for slow and RPC for fast connections. For some magic reason we've never even had to bother our minds with this, things have just *worked*. Lately, though, Outlook clients connected outside of office have started to stall before prompting the user for credentials to proceed with login. And by looking at the Connection Status (outlook.exe /rpcdiag) we can see that for each connection made, it takes significantly long for Outlook to switch from RPC to HTTPS.

So, I understand that this has to do with Outlook judging a connection either a slow or fast, which based on my Google skills seems to be 128Kb / s. What I don't understand is if this before mentioned value is the Adapter negotiated link speed or actually some calculated value between the Exchange/ISA server and the Outlook client? We have not changed anything since the original setup of our infra, so the Outlook acting differently is a bit mind boggling.. By looking at the Connection Status (see the attached image) it has always said Reg/fail being x/1 meaning it has always tried first using RPC then failed over to HTTPS. Now it just seems to take bloody long from it to do the fail over :huh:

Has anyone else struggled with this issue?


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I'll reply to myself since I now know what caused our problem with the RPC over HTTPS function.

Basically my colleague had created a split DNS configuration of our AD integrated zone. Since the clients connected in the local office LAN use RPC and internal AD DNS zone to talk with the Exchange they have no problem, now for clients connected externally through the internet this caused issues, because the same DNS zone resolvable in the office LAN was suddenly resolvable from the internet as well.. just that it was a different server replying to clients which had no idea about the host name of the exchange server clients tried to resolve.

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