I have 3 Win 2003 domain controllers across 3 sites. When one of those domain controllers goes offline connectivity to Exchange is intermittent. When users open Outlook some of them will be able to connect and others won't (seemingly random). All of the domain controllers say that they are GCs which I believe is a FSMO that Exchange cares about. What else is special about this DC that is causing problems with Exchange? This DC is at a different site than Exchange and I would like to get rid of this dependancy so if the site is offline it won't effect the other sites Exchange connectivity.
We have exactly the same problem and it is a "feature" of exchange that it can take up to 30 minutes for Exchange to "notice" that a domain controller has gone offline.
If it is a planned outage then you can switch to using a different one in advance by right clicking on the "Organisation Configuration" node in the Exchange Management Console and selecting the "Modify Configuration Domain Controller" to specify a specific DC.
You will need to do this 30 minutes in advance of the downtime of the DC to allow the setting to take effect. Once the DC is back up set it back to "Use a default" domain controller