When the Internet Mail Service receives an SMTP message, it attempts to resolve the user name in the header against the Microsoft Exchange Server global address list. A user name is the name associated with an SMTP address. If the user exists in the global address list, the SMTP address associated with the user is replaced by the display name for the user's account. Otherwise, the name displays as the SMTP address.
When an SMTP message is received, the Internet Mail Service attempts to resolve the user name in the To, From, and Cc fields from the SMTP addresses. You can increase the performance of the Internet Mail Service by disabling user name resolution.