We are using Exchange 2003 with mailbox servers in 3 locations, with an Exchange frontend server with ActiveSync at the "HQ" location. Everything is connected via MPLS.
If a user's mailbox is located on a mailbox server other than HQ, ActiveSync won't work.
What can be done to resolve this?
Is this a new setup? or it once worked on those other mailbox servers before? You want to make sure your IIS is configured properly on those other servers as well. Please compare the settings to the front working server vs the non-working servers. Activesync with iPhone might have some authentication setting considerations
You can use this to see where it's failing (https://www.testexchangeconnectivity.com/)
You may also want to include how you verify that ActiveSync won't work. It's very vague based on your descriptions
Note: Sorry that this should be a comment, but I don't have enough rep and had to do it through answer
