Consider the following scenario:
- You sign in to Outlook Web App (OWA) and then open Exchange Control Panel (ECP) in a Microsoft Exchange Server 2010 environment.
- You open a large distribution group that contains more than 1,800 recipients, and then you expand the Membership tab.
- You add or remove a recipient in the members list, and then click Save.
The recipient "MailboxName" is already a member of the group "GroupName".