I recommend that you implement split-brain DNS and use the external domain name for everything in Exchange.
That is, create an internal DNS domain for domain.com; create entries corresponding to those in external DNS, using internal IP addresses as appropriate; and then change all InternalURLs and URIs to the external domain. An added side benefit is that
users won't have to remember different URLs internally versus externally.
Ed Crowley MVP "There are seldom good technological solutions to behavioral problems."