In some scenarios there might be a need to configure a so called “Shared Address Space” in Exchange. E.g. a POP3-connector software might be configured to download messages from a POP3-server and deliver them to specific Exchange mailboxes.
If the Exchange users also want to have the external domain as “Reply to”-addresses and/or added as SMTP-addresses on their mailboxes, the domain must be added as an Accepted Domain in the Exchange Organization. When configuring this, it`s important to configure the domain as the correct type of domain:
If configured as the default type “Authoritative Domain”, Exchange think it is the only messaging system being responsible for that domain. This would result in Non Delivery Reports if users tries to send messages to users in the affecting domain, who does not reside in the local domain. In the “Shared Address Space”-scenario, the correct type of domain is “External Relay Domain”. When choosing this, Exchange will first check the local domain to try to locate recipient address. If not found, the message will be routed through the Send-connector closest to the remote domain (usually the internet-connector with the address space *).
This article on TechNet explains the scenario in further detail.