Microsoft Exchange supports system lists, known as global address lists (GAL). A global address list is a group of email addresses that are grouped under a single email address. A distribution list provides an easy way to send messages to a group of people. A message sent to a distribution list goes to all recipients of the list.
When an Exchange account is hidden from the global address list, Messaging systems that have peer Exchange 2010 servers cannot send messages to the accounts. Messaging performs a lookup in the GAL from the user account before sending a message to a user. If the peer Exchange server is 2010, this look up fails when the account is hidden.