Strange access to exchange via web

["Roger Vicker, CCP" <rv-tech@xxxxxxxxxx>]

I have just come across something that leaves me not even sure what 
terms to Google for.

New customer has an Exchange Server 2003 behind a router that only 
allows incoming https. The server is using a self signed certificate. 
The DNS is provided by their ISP, uses a static IP, however the RDNS 
returns the ISP's domain not the customer's. I am working on getting the 
ISP to change the RDNS.

Outside users have been able to use 
https://www.customer.domain/exchange/ to access their email.

Now some (not all) users are getting a "Server not found page." If the 
user does a nslookup on the domain and then substitutes the IP for the 
domain they get the Exchange page. A tracert by name and by IP show the 
same successful results.

At first it looked like the unlucky ones were on a different ISP but I 
have found at least one on the same ISP (even in the same sub-net) as 
the customer that is getting the same failure.

What do I look at next?

Thanks.

Roger Vicker, CCP